Download a backup
On the Team screen, an owner chooses Download backup, confirms with an emailed code, and sets a password. The file is an encrypted ZIP: without the password it cannot be opened.
Restore
Choose Restore from a backup, pick the file and enter its password. It is checked first, then confirmed with an emailed code. The copy it replaces is kept, so you can undo the restore.
Tip
Keep backups somewhere other than the server itself, for example your company drive.