Added into the game at Version 1.3.14, the Skill Swapper item allows for you to change the Skill that your PokĂ©mon has. Not all PokĂ©mon can have their Skill changed by this but you can use the item to change the skill to make the PokĂ©mon a bit more suited to how you wish to play.
When you use the Skill Swapper, the Skill Level will revert down to Level 1 so it'd need to be levelled up again. More PokĂ©mon will be added with new skills as time progresses. This page will detail all the PokĂ©mon that can have their skill change, and what skill it can be changed to.