>> I don't even know how to remove a process from the repository, I can't find any remove button or alike. <<
In order to delete any assets in guvnor, you first have to open this asset. The asset's editor has a toolbar where you should be able to do: File->Delete or File-Archive. If you archive the asset, again in Guvnor navigate to Administration->Archives and permanently delete the asset there if you want.
>>Btw I can't see the new menu item "new work item definition" as described in a video suggested within another topic.<<
Since that feature has been recently added to Guvnor, and no release has happened since, you would need to build Guvnor from master (https://github.com/droolsjbpm/guvnor) in order to see the workitem definition editor right now.