How can update my iMac mid 2011?

High Sierra is the latest version of macOS you can run.

The maximum OS you can update to, is macOS 10.13.6 (High Sierra), if it is already there is no further upgrade.

If it is an earlier OS at the moment, you can upgrade. Look in the AppStore.

Lexiepex wrote:

The maximum OS you can update to, is macOS 10.6.7, if it is already there is no further upgrade.

FWIW, OS X 10.6.7 is Snow Leopard.

You're welcome.

I know, I said so.

Lexiepex wrote:

I know, I said so.

lol 5 minutes later. Yep.