my 2011 Macbook pro OS is 10.13. HOw to upgrade

Your 2011 MacBook Pro cannot be upgraded beyond 10.13.6 High Sierra.

There is no further upgrade path.

As D.I. Johnson says, but some benefit can be had.

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

https://brave.com/

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

Thanks to dialabrain…

FWIW, Firefox 109.0 (current version) runs in High Sierra.

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