You can replace RAM and processors in new iMacs, but there's a catch
 

Earlier this week, Apple revealed a fleet of new iMacs -- and as usual, the "teardown" experts at iFixit have started taking them apart. The first to go under the knife is the new 4K, 21.5-inch iMac (which we've already taken out for a brief test run...

https://www.engadget.com/2017/06/08/...ut-theres-a-c/