Before buying the new RAM that replaces the current RAM of the iMac, it is important that you purchase the right type of RAM. Not only are the modules available in different sizes, but they are also available in different speeds.
To find out the specifics of the type of RAM your iMac needs, click on the Apple icon in the top left corner of the screen and select Info on this Mac. Click on the memory card and you will be shown the type of RAM required.
How to replace the iMac's RAM
Once you've bought the right RAM, you can now replace the old modules with new ones.
The first thing to do is turn off the iMac and unplug it. Then place the iMac face down on a flat surface on a towel so as not to scratch the display.
On the lower edge of the iMac (the edge facing the user), there will be a cover with a small screw in the center. Take a large cross-head screwdriver and unscrew the screw. The screw will not come off completely, unscrew as far as possible and then simply lift the plate.