which iOS can I upgrade to for a MID-2010

Actually, iOS is for iPhones & iPads, macOS is what you need, but you're at the Maximum MacOS now for a Mid 2010 Mac Pro.

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 SW...

Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…

https://www.libreoffice.org/

Thanks to Matti Haveri…

https://discussions.apple.com/thread/254698315?answerId=258771288022#258771288022

LibreOffice 4.0.6.2_MacOS_PPC works in OS X 10.5.8 (last official PowerPC version).

LibreOffice 4.3.7.2_MacOS_x86.dmg (and x86-64?) works in OS X 10.6.

LibreOffice 7.0.4 works in macOS 10.11 and 10.14.

You don't tell us which exactly Mac this is (iMac, MacBook Pro, etc) but 10.14.6 is likely the newest you can run.

There wouldn't be an official upgrade path beyond that.

How to Upgrade  macOS:

The macOS version you can upgrade to depends on the year your Mac was released.

Click the Apple menu () at the top of your screen and choose About This Mac.

Check the Compatibility of your Mac with newer versions of macOS:

➡️ macOS 10.13 High Sierra is compatible with the Macs listed here

➡️ macOS 10.14 Mojave is compatible with the Macs listed here

➡️ macOS 10.15 Catalina is compatible with the Macs listed here

➡️ macOS 11 Big Sur is compatible with the Macs listed here

Only 2010 Mac that can run 10.14.6 with Apple's blessing, is the Mid 2010 Mac Pro.:)