There are several ways to get rid of Salic Law in French History:
1) Kill Robert II of France before the birth of his first child
Robert II of France was the second King of France of the Capetian Dynasty but the only son born to Hugues Capet. If he dies childless, it's likely the nobles will choose his cousin Eudes of Burgundy, who died childless OTL. If so, France's monarchy will remain elective, with the Kings being decided by the twelves peers of France: the Duke of Aquitaine, the Duke of Normandy, the Duke of Burgundy, the Count of Flanders, the Count of Toulouse, the Count of Champagne, the Archbishop of Reims, the Bishop of Laon, the Bishop of Langres, the Bishop of Beauvais, the Bihsop of Châlons-en-Champagne & the Bishop of Noyon.
Problems with that scenario:
- Way too early for what you're looking for
- Women have few chances of being among the candidates... Just look at the HRE: there never was an elected Empress. Maria Theresa of Austria might eventually be considered to have ruled the HRE, but it was her husband Francis I who had been elected Emperor.
- Possibility of Balkanizing France in the way the HRE did.
2) Get rid of Philippe Auguste early on
By this I mean between 1165 and 1187 (before the brith of Louis VIII): if so, the main Capetians bloodline will be dead in the male line which will left the throne open for several candidates.
Problems with that scenario:
- Possible huge war of succession has possible claimants are the Count of Dreux (heir via Salic Law), the Count of Champagne (wedded to the eldest sister of Philippe Auguste) and eventually a Plantagenêt (Henry the Young is married to Margaret of France while Richard Coeur-de-Lion is fianced to Adelaïde/Alys de France)
- Possibility of Balkanizing France between its main nobles or to have France absorbed by the Plantagenêt "Empire".
- Women will not rule: just transmit the right of succession.
- Too early for what you're looking for
3) Have the Direct Capetians die out later than OTL
The best way to do this is to have John I the Posthumous, King of France for five days in 1316 (he was a child that lived as long as such), survive infancy and continue the Capetian bloodline for a longer time. If so, Salic Law will not be established thanks to succession crisis, which will also prevent the Hundread Years' War.
An alternate scenario is to have Louis X of France (John I's father) not die in 1316: this leaves him plenty of time to father sons with his wife Clementia of Hungary.
Whatever POD you use, the result might lead to the Direct Capetians dying in the male line at a time where female succession to the French throne will be easier to accept.
Problems with that scenario:
- A bit early for what you're looking for (1316 and not the 1400s)
4) Have Joan II of Navarra accepted as Queen of France in 1316
Joan II of Navarra was the daughter of Louis X of France and the half-sister of John I the Posthumous. OTL, the nobles chose her uncle to become King instead of giving Joan the crown of France. In an ATL, they could have changed their minds.
Problems with that scenario:
- A bit early for what you're looking for (1316 and not the 1400s)
- Plausibility appears low. Joan was a girl which meant she couldn't lead armies and could marry a foreign prince: the time where she was born is also pretty mysogenistic. Second, Joan was 4 in 1316 and Nobles hate to be ruled by children. Third and last, there were a few rumors that questionned Joan's legitimacy as her mother had been condemned for adultery.
- Room for contestation by other candidates later on
5) Have England win the Hundread Years' War in the 1420s
There are several way to do this: having Henry V living long enough to secure the conquest of France or have Joan of Arc fail/not appear are amongst the possible options.
Problems with that scenario:
- Result in an Anglo-French Union, which might not be what you want.
- Might not get rid of Valois claimants: Charles VII might face a tougher challenge, but that doesn't mean he won't fight for his throne and leave heirs.
- Successoral law relatively unclear because of how events went in England (with Richard II being overthrown and replaced by a bloodline that wasn't forcibly the senior one).
6) Have Henri IV fail to claim the French throne
Henri IV's succession was heavily contested by the Guise and the Spanish because he was Protestant. It was also contested by some because of the weak bloodties between Henri III and Henri IV (only common ancestor being Saint Louis, who had rule 300 years prior).
If Henri IV hadn't convert, he might not have won the French throne. However, the best way for Henri IV to fail claiming the throne is for the Spanish and the Guise to find some agreement: they tried to use the States General to abolish Salic Law, but failed because they couldn't agree on the candidate (the Guise wanted the future Henri II of Lorraine (who was a relative of theirs as well as a nephew of Henri III) while Philip II of Spain wanted his daughter Isabella (a niece of Henri III)), because the State General's delegates didn't wish to bow to the Spanish and because Henri IV finally converted to Catholicism and got rid of the main problem. If the Guise and Spanish play their cards better, then Henri IV might not become King of France.
Problems with that scenario:
- The Religious Wars will turn into a Civil War. Henri IV will not easily give up the possibility of becoming King of France and will still have supporters, who might not be all Protestants (Other Bourbons & Capetians-related Princes supported Henri IV because getting rid of Salic Law also killed their chances of getting the throne...). Plus, it's not like the Guises or the Spanish will be easily accepted.
- Maybe a bit late for you (it's the late 1500s)
And I'll stop there because the other possibility are either too far away for you or have the problem of Salic Law being "stone-carved" because it's been accepted and is a
Loi fondamentale du royaume (I think) which the King can't change on a whim or without consent. Plus, on several points, that law seems pretty handy as it effectively reduces the chances of Personnal Union were France is not dominant and of a foreign ruler ending up as King of France (which was the main reason we had the Hundread Years' War).
