Booted 2013 imac using 'retired' internal drive, but don't remember password to use OS

Retired the 1tb internal disk in my 2013 imac 7 years ago as i was having minor issues. To avoid dealing with potential problems I purchased an external ssd drive and set it up as the boot drive. All has been fine though today I decided to revisit the internal drive. It boots up fine, I get I what I believe is the Sierra icon and of course the banner into which I am to type my 7 year old and long forgotten password. How do I update a password to let me into/on the computer using the old drive. I can get on my current OS (Catalina) that's on the ssd drive. Any assistance would be much appreciated.