can I update my 2013 iMac to version OS X 10.11 ?

You can upgrade a 2013 iMac to 10.11 through 10.15.

How to get old versions of macOS - Apple Support

Check compatibility, and get macOS upgrades here: How to get old versions of macOS - Apple Support.

The last version of Mac OS a 2013 iMac can use is Catalina, 10.15.7.

Can you please tell me how? When I go Software Update, its comes back with No Updates Available.