How can I update the OS on my old iMac? It is running OS 10.13.6

If the computer is a 2011 or earlier then 10.13.6 is the latest version of Mac OS that will run on your iMac. However if it made after 2011 please refer to How to get old versions of macOS. Check to see the system requirements for each of the OS's too and then you will have an idea of what your computer can be upgraded to.

What year model is your iMac?

see > Identify your iMac model - Apple Support

For macOS hardware requirements,

see > How to get old versions of macOS - Apple Support

Thanks. Unfortunately 2011

Then it may be time for a new Mac.