how can I upgrade my OS

None.

(187661)

Yes. Sierra and above require a more-recent model.

(187662)

I have a MacBook Pro retina, 13inch, 2015 with 8GB. Which of the following updates are the best for me? High Sierra, Mojave or Catalina. Thanks.

Mojave if you need 32-bit software and Catalina otherwise.

(187716)

My mac has 120GB total memory. Is this relevant to my choice? Is there some kind of limitation?

No, it's not, but you'll need to have a portion of that free to upgrade or in regular usage.

(187717)

Thanks a lot. You are very helpful.