I want to update my mac from 10.11.6 version

If your computer doesn't meet the requirements for Mojave that are in that article, see if High Sierra will work.

How to get macOS High Sierra   

Check to make sure your applications are compatible.      

Application Compatibility

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

It depends on the specs of your Mac.

macOS - How to Upgrade - Apple

Check in Apple menu > About this Mac for the year of your iMac.