iMac (Retina 4K, 21.5-inch, 2017) , can I upgrade my memor?

Hi amru258,

Unfortunately, you cannot upgrade your memory. Only an Apple Authorized Service Provider can.

Install memory in an iMac - Apple Support

However, why would you like to upgrade your memory? If it's due to performance issues, it's probably not a memory issue. To check, I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as shown here: How to use the Add Text Feature When Post… - Apple Community. We will then examine the report for issues in your Mac.

Cheers,

Jack

Jack-19 has given you some sound advice. in most cases, I would recommend professional help to upgrade the memory in your particular iMac. however, some people have the skills to attempt the upgrade themselves. you can have a look at OWC's memory DIY install video to see if this is something you are capable of doing.

please see https://eshop.macsales.com/installvideos/imac-21-inch-2017-memory/