iMac (21.5-inch, Mid 2011) needs upgrade

The RAM (memory) can be upgraded to as much as 32GB and the storage (hard drive) can be upgraded.

However there is no hardware upgrade that would allow the macOS to be upgraded beyond High Sierra 10.13.6.

If you wish to continue usingit in this day & age..

Mostly you just need a more modern Browser than Safari on Catalina, Mojave, or High Sierra like Firefox or Brave.

https://brave.com/

https://www.mozilla.org/en-US/firefox/83.0/system-requirements/

Firefox 83.0 System Requirements (mozilla.org)

Opera…Thanks

https://www.opera.com/download/requirements

Opera system requirements | Opera

Thanks to dialabrain…

Firefox 115 is now the current version. runs in High Sierra.

119.0.1 is now the current version and requires 10.15 or later.

https://www.mozilla.org/en-US/firefox/new/

Thanks to A Brody, Browsers by Mac operating system…

Browsers by Mac operating system - Apple Community

If you need office software, Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…

https://www.libreoffice.org/