memory

You have to take it to an Apple Authorized Service Provider because Apple will not do upgrade work. Some Apple Authorized Service Providers will not work on the sealed-case iMacs like the 2017 models due the the cost and difficulty. If you find one willing to do the work, expect to pay for two hours of labor at an estimated US$75/hour in addition to the cost of the RAM and reseal kit.

⚠️ However about 95 percent of "slow iMac" reports here since 2012 turn out to have nothing to do with RAM and everything to do with the slow and under-spec hard drives Apple installed in entry-level iMacs. You can throw hundreds of dollars of RAM into a computer with slow hard drive and change nothing.

Also, one of the 2017 21.5-inch iMacs is the crippled "education/institutional model with a slow older-style processor.

To properly assess if your had the crippled "Edu-model" and/or if you have the common hard drive issue, please post a snapshot of your system configuration. Fortunately there is a safe, secure way to do that.  We can quickly and within the confines of these forums help you determine what issues are at play if you use EtreCheck Pro, available here:

https://etrecheck.com/index

The free version will do nicely for this purpose, although the app is worthy of our financial support.

We can see hard data about drive performance, software issues, and RAM usage. Etrecheck is the development of a long-serving and trusted contributor here expressly for displaying information in these forums to help us help you. It will not reveal any personal or secure information.

Run it, select “Report" from the left-hand pane (scroll down to the bottom of that pane to find):

When its report displays, click the "Share Report" icon from EtreCheck’s toolbar and then "Copy report” from the resulting dropdown.

⚠️ Please DO NOT highlight the text in the report before using Etrecheck’s “Copy report” command—that will garble the formatting and make the report slower and harder to evaluate.

NOTE: Changes in late 2018 to the forum software require you use the “Additional Text" icon (see example below) to embed the report into a post:

Paste the report into the resulting “Additional Text” window:

Our seeing an EtreCheck report commonly letts us give yo usloutiosn inonly one more post, much better than our playing a protracted game of "20 Questions" over several hours of days.

The 2017 21.5" iMac must be completely dissembled to upgrade the RAM.

You will need to call around, because not all AASPs can or will do that complex of an upgrade.