Skill Trainers can raise the maximum level of character's skills, and reset selections (allowing you to reallocate skill points), for a fee.
Divinity II: Ego Draconis[]
A skill trainer for the Dragon Knight's Battle Tower is selected on Sentinel Island during the quest The Prophecy.
(Note: If pitted against one another during the selection, Hermosa will win and kill Kenny, making him unavailable for selection.)
Divinity II: Flames of Vengeance[]
Although you still have access to the Battle Tower, two new skill trainers are found at the Ministry in the Great Market of Aleroth. They offer training in magical, physical, and utility skills.