can an iMac (21.5 inch late 2012) memory be upgraded?

i agree with Neil, it's not something most users can do. but on the flip side, the iMac would be open so you could also upgrade the HDD to an SSD at the same time. the SSD would dramatically increase the speed. much more than a RAM upgrade alone.

here's what it involves for both operations: iMac 21.5-Inch (late 2012) OWC Install Videos

Yes, but the process is difficult and shouldn't be carried out by regular customers.

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and i forgot to add, you could of course have it done professionally.