So the battle of Phillippi was the last stand of the "Liberators" the conspirators who assassinated Julius Caesar. They were up against the second triumviirate, lead by Marcus Antonius, and Octavian. The first battle resulted in a draw more or less other than the suicide of Cassius. Brutus was less experienced than Cassius, and when the final engagement came, his attack was repulsed.
So what if, on the first battle, instead of being a draw, Brutus and Cassius gain a victory. On the same day as the first battle of Philippi, the Republican fleet is able to intercept and destroy the triumvirs' reinforcements (two legions and other troops and supplies led by Gnaeus Domitius Calvinus) as they did IOTL.
In a much worse position than they were already, the triumvir's troops morale is at its lowest ebb, and when the second battle eventually comes, it is a complete victory for Brutus and Cassius. Antony, Octavian, (and if you want, Agrippa) are killed or committ suicide.
What effects would this have? Lepidus is still in Italy mind you, so the triumvirs aren't completely destroyed, but you have to think the political situation in Rome once the defeat of Antony and Octavian reach the city, will be too much for Lepidus to handle.