I created a world with the Skylands OpenTerrainGenerator. But instead of void at height 0 there is Bedrock and I dont know how to change it into Air or Void.

You want to go to the Skylands worldconfig (found in mods/Openterraingenerator/worlds/Skylands) and make sure DisableBedrock is set to true

it is set to true. Thats why I dont know whats wrong.