How do I upgrade memory in iMac (Retina 4K, 21.5-inch, 2019?

RAM on that model is not considered user-accessible. Changing RAM requires essentially a complete teardown and gutting of the computer to access the well-hidden RAM slots. And the case is sealed shut. Some Apple Authorized Service Providers won't do RAM upgraded on sealed case iMacs like yours

The better approach is to tell use why you want more PRAM. The vast majority of "slow 21.4-inch iMac" complaints here are not RAM-related but rather due the slow and under-spec mechanical hard drives Apple opted to use as their base storage option. Adding RAM will not a detectiable difference in felt performance.

A simple test lies in these three questisons:

• • Is the compteur slow to boot?
  • Are apps slow to launch?
  • Once launched, do apps seem to run fine?

Three YES answers strongly suggests the well-known slow-drive issue.

The Memory (RAM) in that model is not user upgradable.

see > Install memory in an iMac - Apple Support

That iMac model must be dissembled by an Apple Service Provider that is willing to do upgrades.

The usually workaround is an external solid-state drive but before spending any money, it is prudent to post a system config snapshot so we can see if any other issues are causing the "slows." Fortunately there is a safe and secure way to do that.

We can quickly and within the confines of these forums help you determine if software issues are the problem if you use EtreCheck, available here:

https://etresoft.com/welcome

We can also see hard data about drive performance 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) 

Then 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: Dec 2018 changes 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.

Also see this excellent user tip on posting text reports like EtreCheck. It is better illustrated than mine.

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

Hi, this sounds exactly like my issue - is there a solution to the' slow drive'?

The RAM in that Mac is non user-accessible. You would have to find an Apple Authorized Service provider willing to do it.