It is common to have buyer's remorse after spending skill points in The Witcher 3: Wild Hunt. Thankfully, CD Projekt Red thought of this, and came up with a way for you to respec your character without restarting the game.
In order to respec you need to acquire a Potion of Clearance. This item "Returns all spent Ability Points to the available pool so you can redistribute them". Unfortunately, obtaining one will cost you a sizeable amount of Crowns, but at least there's an option. Below I'll go over how you obtain one of these sacred potions.
There are two vendors that sell the Potion of Clearance. The first you can gain access to is Keira Metz in Velen. She is found in a cabin just Northeast of Midcopse, a town on the West side of Velen. If you have trouble finding her house, follow the road above Midcopse and take the first right. Her house is just ahead. Note that you will need to complete quests for her before she will sell you a Potion of Clearance for 1,000 Crowns.
The second merchant is in Novigrad. The merchant is in a building located in the northern Plaza just Northeast of Hierarch Square. Once in this plaza, enter the building on the West side. This merchant will sell you the Potion of Clearance for 1,000 Crowns after you talk to him about an "artefact (the top dialog option).
Once you've reset your skill points, check out my Build Guide to get a better idea of which skill points are most effective.