How can I upgrade my memory? I now have 8 GB (iMac 21" 20170

Apple released two 21.5" iMacs in 2017. Both have two sealed-in, non-user-accessible RAM sockets. According to MacTracker, they are:

• A non-Retina model (iMac18,1). Takes 260-pin PC4-17000 (2133 MHz) DDR4 SO-DIMMs. Official limit: 16 GB of RAM; actual limit: 32 GB.
• A Retina 4K model (iMac18,2). Takes 260-pin PC4-19200 (2400 MHz) DDR4 SO-DIMMs. Official limit: 32 GB of RAM; actual limit: 64 GB.

I would suggest checking the Memory Pressure graph in Activity Monitor while running a normal workload to see if you need a RAM upgrade at all. Slowness on that machine may well be due to something else – especially if your internal drive is a 1 TB notebook HDD (which runs at 5400 rpm), or a 1 TB Fusion Drive (which combines the same slow hard drive with a skimpy 32 GB SSD).

If you go ahead with the RAM upgrade, you may want to buy the RAM from Other World Computing or from Crucial, and to have it put in by a repair shop. Macs can be picky about RAM – and for many, OWC and Crucial are the go-to suppliers. Tearing apart a 2017 21.5" iMac to get to the RAM sockets, and putting it back together again, might well be more than you want to handle, which is where the services of a competent repair shop would come in.

Computer is slow, even for the finder.

Very familiar clue, and one that suggest RAM is not your issue.

Based on thousands of reports here, "slow" in a 21.5-in a iMac made between 2012 and 2019 and with the entry-level storage options, will be due to the slow mechanical hard drives that Apple selected as base storage.

If:

—the computer is slow to start up (boot), and

—apps are slow to launch, but

—once launched, seem to run acceptably fast....

...then you are suffering the slow hard drive curse. No amount of extra RAM will change that.

That is based on history. If you want a data-driven evaluation of your specific issue in this setting where we can neither see nor touch your computer, please post an EtreCheck report. We can quickly and within the limitations of these forums help you determine what issues are at play without our playing a protracted game of "20 Questions" with you that could go on for days. 

EtreCheck Pro is 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 interferences, and RAM needs if any. Etrecheck is the development of a long-serving and trusted ASC contributor. It is a reporting app, not a "fix-it” app, expressly for displaying information in these forums to help us help you remotely. It will not reveal any personal or secure information.

Please see this excellent user tip on how to post long text reports like EtreCheck's into a forum response:

How to use the Add Text Feature When Post… - Apple Community

that model does not have removable memory modules

Apple iMac (21.5-inch, 2017) iMac18,1 | SSD Upgrades | Crucial.com