can i update my imac 21.5 inch?

A 2013 21.5" iMac 14,1 model can run up to macOS 1015 Catalina.

see > https://support.apple.com/en-us/HT210222

The 2013 21.5' iMac's RAM and other hardware is not user upgradable.

see > https://support.apple.com/en-us/HT201191

Update what and which model year 21.5 " iMac do you own, they were available from late 2009 to 2021 in MANY configurations so we need you to be much more specific.