Hi Pauline,
The Mods redacted your serial number for security reasons so some of us do not know exactly which iMac you have. What exact sub-model shows in "About this Mac...", in your Apple menu.
The comments posted here are pertinent to iMacs made in 2012 and later. 2011 and older 21.5-inch iMacs have user-accessible RAM/memory.
If you have a 2012 or later iMac 21.5, note that Apple intimates that some Apple Authorized Service Providers will not attempt that difficult upgrade in this Apple article on the subject:
Install memory in an iMac - Apple Support
with this statement:
* Memory is not removable by users on iMac (21.5-inch, Late 2012), iMac (21.5-inch, Late 2013), iMac (21.5-inch, Mid 2014), iMac (21.5-inch, 2017), iMac (Retina 4K, 21.5-inch, 2017), and iMac (Retina 4K, 21.5-inch, 2019). If the memory in one of these computers needs repair service, contact an Apple Retail Store or Apple Authorized Service Provider. If you’d like to upgrade the memory in one of these models, an Apple Authorized Service Provider can help. Before you schedule an appointment, confirm that the specific Apple Authorized Service Provider offers memory upgrade services.
The upgrade requires almost a complete gutting of a 21.5-inch iMac. A provider I know says he has to charge two hours labor at US$80 per hour to accomplish the upgrade in addition to the cost of RAM modules and the resealing kit. Some clients do not want to invest the extra $160+ to get a RAM upgrade.
