How to I update my iMac (21.5, Mid 2011) from OS X Yosemite to OS X Big Sur?

Please refer to How to get old versions of macOS which means, in short for a 2011 High Sierra 10.13.6 is the end of the line. If you want or need Big Sur you need a new Mac. Please refer to macOS Big Sur - Technical Specifications and you will see.

Welcome to the Apple Support Communities.

You can't; but you can upgrade to macOS High Sierra. Get it here: How to get old versions of macOS - Apple Support. Backup first!

Jack