well, it's already broken no ? 😅
If you can easily re-run your import, I'd suggest to fix the structure in the old software, then import again.
If there's no way to go back, you will unfortunately need to edit directly in the database. You could set all parent relationships to NULL, that way all tags will appear in the Flarum editor and you can move them at will.
Keep in mind that moving tags in the Flarum editor won't change which tags are applied to your discussions. So if you're completely reworking the structure, your existing discussions will likely have tags that don't match the new structure.
Can you let us know which forum software you're migrating from and which tool you used to perform the migration ? This might help us to give advice.