Improve the weather leading up to the Battle of Slivnitsa (i.e. stop it from raining too much). Serbian forces aren't bogged down, they can attack earlier, and the Preslav regiment doesn't have time to arrive to reinforce the Bulgarian side. The war may last longer, but a victory at Slivnitsa may turn it in Serbia's favor.
As for what would happen, Bulgaria would not unite with Eastern Rumelia (which would remain part of the Ottoman Empire) and the House of Obrenović may retain the Serbian throne. Serbia might gain territory in the western Balkans, if the Austrians keep their promises, and Serbia may end up remaining pro-Austrian. There's a good chance this could butterfly (or at least delay) WWI.