OS Mojave install- locked hard drive due to backups

Hi,

I am trying to update my OS to mojave on an IMAC late 2013- when the options for choice of drive come up ,the Mac hd is locked due to the fact that it has been used as a backup disc. I didn't think i had backed up to the Mac hd as i use a time capsule??

Is there a setting i should change?

Should i delete the backup?

How to unlock the drive to download the OS update?

Thanks, Jason.