You can literally download windows 10 from Microsoft website. There's no need for a key or activation as it will be linked to your account. That's it really.
Backups can get stuck when the old data is corrupt. You can go online to onedrive and view your backups and delete the old one. You'll then be able to backup with a fresh copy.
How old is the phone? If it's under 24 months, you cam send the phone away for repair. The button is probably not...