I’m not sure what exactly you’re referring to, but I can give you a quick guide on how to evolve a fighter.
First, you need to have maxed out the level of the fighter that you want to evolve. With bronze this is 30, Silver is 40 and gold is a whopping level 50, but you can’t evolve golds (yet) so just focus on silver and bronze. Then when you are sure you want to take that fighter from their current rarity to the next you press evolve and choose 3 or 4 other fighters of the same character and same rarity level (bronze/silver) that your card already is to start with. This will evolve the fighter but put them back to 0 XP at level 1. Your skill tree remains completely unchanged though. The only difference is the core stats of the fighter. All upgrades still cost the same and you keep any you had before.