Q: TO uncheck selection of encrypt iphone backup

There is no workaround. You have to Restore the phone as new.

About encrypted backups in iTunes - Apple Support

How can I turn off iTunes backup encryption?

To turn off iTunes backup encryption, enter the password and uncheck the Encrypted backup box in iTunes. Your encryption password is always required to turn off backup encryption.

You can't use an encrypted backup if you don't have the password. If you forgot your password, the only way to turn off backup encryption on your device is to erase your device and set up as new. Erasing removes all data from your device. If you don't want to erase your device, restore from an iCloud backup instead.