Guild Wars 2

How do you reset skills, if you picked one by mistake?

I guess that would be slot skills.

Once a skill has been ‘bought’ you can’t trade it back – so if you’ve spent skill points on the wrong skill you will just have to accrue some more.

If you are talking about swapping out skills on your skills bar you need to be out of combat and then use the little arrow at the top of each skill to access those available to you – or do it through the hero screen [H] where you can see all of the skills available to you.

If you mean spending a skill point to unlock the skill that cannot be undone, so just save up more skill points and get the one you want.
If you mean that you put the wrong skill into a slot then you can just swap it out for another slot skill.