How do I respec a follower in Diablo 3?

Note: this answer is out of date in version 2.0 and above.

You have to right click their portrait in the main window, not the follower window (default hotkey F). There will be an option called "Retrain" which will let you redistribute her skill choices. This is absolutely free and has no cooldowns or anything like that, by the way.

As of 2.0.1. (maybe even few patches earlier) there is no more retrain button,you can now freely choose skills for your follower.

Meaning,if you don't like the chosen skill,you can simply click on the other one to activate it. Changing skills has no cooldown and can be done anytime,even in combat. But it won't reset the actual cooldown on skill to prevent abuse.