raI keep an iMac 24 inch fully operated with OS X EI CAPITAN version 10.11.6 in what new software can I upgde it?

As mentioned, that OS is the highest yu can run n it, but there are better Browsers...

https://brave.com/

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

https://www.libreoffice.org/

Apple macOS (Mac OS X)

The software and hardware prerequisites for installing on a Apple macOS computer are as follows:

• LibreOffice 6.3.x: macOS 10.10 (Yosemite) or higher
• LibreOffice 6.2.x: macOS 10.9 (Mavericks) or higher
• Intel processor
• 512 MB RAM
• Up to 800 MB available hard disk space
• 1024x768 graphic device with 256 colors (higher resolution recommended)

The last 24" iMac was from Early 2009. OSX El Capitan is the latest version it can run.