How to update an old iMac

Which Mac is it? (Model name and hardware mode year / time of year, from About This Mac) That will determine how far you can upgrade macOS, if you can upgrade it at all.

If this is an iMac with a hardware model year of 2011 or earlier, you cannot upgrade it past macOS 10.13.* (High Sierra).

A big +1 on the "what iMac ? requests.

Apple has made computers called iMac continuously for abut 27 years. This is what "About this Mac..." looks like in macOS 10.13:

We need the SUB-MODEL including the year to effectively help in this setting where we cannot neither see nor remotely access your computer.

Thanks!

FWIW, you need a Late 2012 or later iMac to update beyond High Sierra 10.13.6