restore iphone backup

See this support document for an explanation of encrypted backups. iOS: Troubleshooting encrypted backups - Apple Support

Then see this one explaining what you are going to have to do if you have an encrypted backup and cannot remember the password. Create and delete iPhone, iPad, and iPod touch backups in iTunes - Apple Support

I do not know what to do.

You cannot used the encrypted backup without knowing the password.

Try to remember, if you don't, this is a security feature and cannot be bypassed.

If you can't remember the password and want to start again, you must perform a full software restore and choose set up as a new device when iTunes prompts you to select the backup from which to restore.

You're welcome.

Best you use computer iTunes to delete your iPhone backup.

iTunes > Edit > Preferences... > Devices > select the backup and click the delete button.