I unwrapped everything automatically and it endend up being really bad for painting. How can I delete the unwrapping and start again?
You can start again and re-do the UV unwrap just by doing the unwrap command again. In edit mode press U and select any of the methods. The UVs will be redone with the new method and the old UVs will be discarded. The mapping methods are explained here.
There is one exception to the above and that is UV pinning. If any of the UV vertices are pinned (they are red), you need to unpin them first with Alt+P. Pinning prevents UV vertices to be re-done.
Deleting the UVs is another thing. One mesh can have multiple UV unwraps and you will find them in Mesh Properties:
You can delete any UV map completely with the - button.
If there is no UV map awailable, the unwrap operator U does create one automatically when unwrapping.