Hi @sven_wedig and welcome to the forum!
You can accomplish what you want by dragging and dropping the subtasks as illustrated here:
However, note that are are two variations of this process, depending on whether you want the subtasks to remain as both subtasks and also show as top-level tasks, or if you want to remove them as subtasks and just move them to be only top-level tasks (from your post, I suspect you want the latter of these options):
If you want them to remain as subtasks and also show as top-level tasks, drop them directly onto the name of the section.
If you want to remove them as subtasks and only have them show as top-level tasks, drop them anywhere other than onto the section (in this case, when you drag them, you’ll see a thick horizontal line showing where they’ll end up when you drop them).