In Diablo 3 for the PC, you can change the skills of your companions at any time for free, and you don’t even have to go back to town to do it.
This is a nice feature, because it allows you to play around with different play styles. You can switch your Templar from melee DPS, to tank healing before a boss, etc.
All you need to do is right click on your follower’s portrait in the upper left hand corner of the screen. One of the options there should be Retrain, which will reset all of their skills. Hit the F key (assuming you have it mapped as the default button still for the Follower inventory screen) and select the skills you want them to have.