Can I upgrade my Mid 2011 iMac?

High Sierra is soon approaching an age when it likely won't see any security updates anymore (it did get one 2 months ago). I do suspect it won't see any actual system updates or bug fixes anymore. What it does say is your computer can't run a newer system that what you are already running so soon your computer will be pretty much running all it will ever be able to.

If you mean operating system version, no, not if you are running High Sierra already as your profile indicates.

Identify your iMac model - Apple Support

iMac (27-inch, Mid 2011)

Model Identifier: iMac12,2

Part Number: MC813xx/A, MC814xx/A

Newest compatible operating system: macOS High Sierra 10.13.6