my 2011 Macbook pro OS is 10.13. HOw to upgrade

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/

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

There is no further upgrade path.