Backup as mirror not just adding to destination

I’m backing up my photos from an external HDD to My Passport HDD. When I backup it adds the new files to my destination folder but does not remove files I’ve deleted from the source folder. How do I get it to mirror source folder when I backup

The backup is working as intended. A backup (A secondary copy available as an emergency in case the primary copy is not accessible) is not meant to delete files, but to copy/duplicate them.

If you would like for your files to be kept in a 1:1 ratio (Copied files will be deleted when removed from the original source) then you would need a synchronization software as opposed to a backup software.