Sounds like you have a 2015 iMac, 21.5-inch display. Available in Retina and non-Retina versions.
The answer is the same for both versions though - the RAM chips are soldered to the logic board. No slots to upgrade. In theory a repair shop could desolder and replace the RAM chips individually. Probably cost more than the machine is worth today.
Among the various 21.5 models, RAM slots are present up through 2013, and reappear in 2017. 2014-2015 models were soldered. Thanks, Apple.