I ran a backup of apps and settings to the cloud. Since then, I suddenly ran an app which downloaded all sorts of spam and links to win all sorts of expensive stuff. I need to get the phone back as it was before this happened. How do I do this?
Uninstall the app? You cannot get any virus or anything on Windows Mobile, so simply deleting an app you installed will get the phone back.
To restore a backup, you go into Settings/About and choose to Reset the phone. Re-login as your Microsoft account at reboot and it will offer to restore the backup.
NOTE: Make sure your photos and videos are backed up first and the backup does not back those up. Connect to a PC or use OneDrive etc. to do that.