Adding Mac serial number to device

You cannot add the SN information, it should already be showing though. Please click the Apple symbol in the upper left of the display and then click About This Mac. Do you see the SN?

If for some reason you do not that would indicate the main logic board was likely replaced at some point and the SN wasn't entered by the repair shop. That means you need to take it to your local Apple Authorized Service Provider to have it serviced and they can enter the SN info, you do not have the correct tools available to you to do the work.

The original serial number is underneath the stand.

pcl20 wrote:

I would like to know how old it is and if I can update it ti Ventura OS

When you go into About This Mac in the Apple () menu, it should tell you the Mac's model name and hardware release year, e.g. iMac (27-inch, Late 2012) or iMac (21.5-inch, Retina 4K, 2019).

The hardware model will determine whether you can upgrade to Ventura. For an iMac, it looks like you need one whose hardware release year is 2017 or later. (Other than a 24" M3 iMac, which is recent enough that it probably can't run anything earlier than Sonoma.)

macOS Ventura is compatible with these computers - Apple Support

If you can upgrade, see

How to download and install macOS - Apple Support

If the S/N is missing from the About This Mac display, then someone other than Apple has replaced the motherboard, as Apple techs would burn the new motherboard S/N into the firmware allowing it to show on the mentioned display. This is not a process that can be done by the end user.

The serial number on the stand will be correct. That number would also be the number that should have been burned into the replacement logic board when the Mac was serviced.

[Redacted as a duplicate no thanks to the quirky hosting software … /viking]