Hi can I upgrade my Imac from OS X El Capitan 10.11.6

The upgrade is available by Mac model and year, not by what macOS the Mac is currently running.

Most any iMac 21.5” model can get to at least macOS 10.13, with more recent iMac 21.5” model years supporting more recent macOS versions.

Here is how to: How to download and install macOS - Apple Support

Have a backup prior to the upgrade. That’s the only easy path back, in the unlikely event that the upgrade should fail.

What year model is your iMac...?

see > Identify your iMac model - Apple Support

To update an iMac to the latest available macOS version

see > Update macOS on Mac - Apple Support