If you mean weapon skills, you can't...its unique for each weapon, 1-3 main hand, 4-5 off hand.

For the last 5 slots, you can change them anytime as long as you're not in combat so you should switch them to suit your encounters

I suppose you mean you wanna undo the skills you've unlock? That's not possible, but you shouldn't worry bout that, skill points are unlimited, even after you've reached 80 you'll still get skill point everytime you fill up the exp bar..