Can I update an iMac mid 2011 model to Big Sur?

Make sure you do not already have an older version of the High Sierra upgrade package in your Applications folder before downloading. The presence of an older versions and trigger error messages.

What system software version are you now running?

No sorry, High Sierra 10.13 is the highest that a Mid 2011 iMac can run.

Go to > https://support.apple.com/en-us/HT211683 and try the High Sierra links in that support article.