i have a mid 2011 iMac running High Sierra 10.13.6. how can i upgrade?

You are limited by the hardware, versions of Mac OS after 10.13.6 require GPUs that support Metal. Before you ask, no you cannot upgrade a 2011. If you want later versions of Mac OS it is time for a newer Mac.

You can’t upgrade anymore. macOS High Sierra is the latest/last supported version for your Mac.

Cheers,

Jack