You can buy an Orb of Regret from an NPC for 2 Orbs of Scouring.
Orbs of Regret are quite expensive. You should get enough Respec points from story quests for minor fixes to your skill tree. If you want to respec more than 30-40 points, it might become worth it to just reroll and make a new character.
If you sell a gem to an NPC along with an Orb of Regret, its gem level will be reset to 1.
There are vendor recipes to reset your Act 2 bandit choice with Orbs of Regret.