I have an older MAC Mini (Mid 2010) which currently has OS X El Capitan ver. 10.11.6. What's the latest OS I could upgrade it to?

I have an older MAC Mini (Mid 2010) which currently has OS X El Capitan ver. 10.11.6. It works great but I'd like to know what's the latest OS I could upgrade it to? Currently having problems viewing videos (especially on Youtube), and loading certain WEB pages, and think updating my OS would solve this problem. It has a 2 GIG backup hard drive, so doing the backup should be secure. Can anyone help me with this?