I would like to know the answer to this as well. I understand we can do this in pre-processing. However my labels didn’t import correctly, and I would like to fix them permanently.
There isn’t a way to change or delete classes at the moment. If you just need it fixed for training, like @DuaneNielsen said, you can do it in pre-processing.
If you need to fix them permanently, the easiest way right now is to remap or exclude classes, export the dataset with no augmentations, then import it back into a new project.
It’s a bit annoying, but that’s how you can do it if you really need to, at the moment.