I want to upgrade RAM

in your specific iMac, upgrading the RAM IS NOT a user friendly operation. it is actually EXTREMELY difficult. your best bet would be to have it done by a competent professional.

to see the difficulty level, you can have a look at this page: OWC Memory UpgradesfFor 21.5" iMac with Retina 4K & Non-Retina Display (2017) and scroll down to view the self install videos.

as an afterthought, why do you feel you need more RAM? if you are experiencing a slow Mac? it may be due to the drive configuration in the Mac. is it SSD based? or do you have an HDD or Fusion Drive? if HDD or Fusion based, that could explain the slowness.

It may be better to run EtreCheck which will provide us with some details about your computer and its usage including memory usage and summaries of system logs as well as sometimes identifying a slow or failing drive. In addition to possible drive issues, third party software is just as likely to cause performance issues. Post the complete report here using the "Additional Text" icon which looks like a piece of paper.

You can check Activity Monitor's memory tab for memory usage. While using the Mac under its heaviest workload, If the Memory Pressure is not red and the "Swap Used" and "Compressed" do not show GBs of RAM, then you have enough memory. If the Memory Pressure is yellow, then you may be borderline on needing extra memory.

on running Etrecheck