How to delete Big Sur off of external HD?

Had insufficient space on my MacBook so I downloaded Big Sur onto an external HD that I use. Ended up being unable to even login operating on Big Sur, it would just beach ball endlessly in the login screen.

I shut off my computer and unplugged my HD. Restarted without HD plugged in, and now back on Catalina. Plugged HD back in, and now it's just labeled as "Update" in finder as opposed to the name I chose for it. It still has all my original content on it, but I'm unable to see any of it in the HD screen. How do I entirely remove Big Sur? When I click 'eject' on Update I get the following message “Update” is a volume on a disk that has 2 volumes. Do you want to eject “Update” only, or both volumes? with the options 'Eject' 'Eject All' and 'Cancel'.