exp points if ur asking about dimensons
and if your taking about the anime then theirs 2 ways
1.when a bakugan wins a brawl they absorb a little bit of their opponets energy and if they win enough that energy builds up then they evolve
and the other why is
2.if a bakugan absorbs a "Power Object" Like a Core or a Attribute Energy they will evolve but the Bakugan would of had to evolve at least twice useing the first method to evolve this way.