I'm running OS EL CAPITAN 10.11.6 and need to upgrade my operating system to work with Adobe CC. How do I upgrade

See if you can upgrade to 10.12, 10.13, 10.14, or 10.15, see sys requirements...

https://support.apple.com/kb/sp742?locale=en_US

https://itunes.apple.com/us/app/macos-sierra/id1127487414?ls=1&mt=12

https://support.apple.com/kb/sp765?locale=en_US

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

https://support.apple.com/kb/SP777?locale=en_US

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

Don't use Firefox, try Safari...

https://itunes.apple.com/us/app/macos-sierra/id1127487414?ls=1&mt=12

What does my link take you to?

Great to hear, take care my friend. :)

If you get a corrupted message...

Naw, not damaged, it's just that you have an old copy already in Applications, Trash the Install MacsOS High Sierra.app in Applications then a new copy more recent version can be downloaded.

If you get a corrupted message...

Naw, not damaged, it's just that you have an old copy already in Applications, there are 2 cures...

Disconnect from the Internet & set the Date back to 2014, then you can run the Install MacsOS Sierra App in Applications, so expired Certificates are not expired.

Or... trash the Install MacsOS Sierra.app in Applications then a new copy can be downloaded.