Is there a way i can change a biome at a location after the world is already generated?
WorldEdit has a //setbiome command 
(Edit) Wait … this is the development section … ok i don’t know currently if there is something in the API, sry xD
Ok, second try:
Since a World is an Extent and an Extent is a MutableBiomeVolume you should just be able to do world.setBiome(position, biome) ^^’
(If you test it, make sure you reconnect because otherwise the changes wont be visible)