This is a very, very popular upgrade as the value you get from doing this yourself is often a significant $ savings VS buying the iMac with more RAM already fitted inside.
As per usual, we understand that many people don't have a workshop, so I'll be doing this on a plain kitchen bench with some white paper on top. You can generally upgrade quite safely by first touching a grounded object (I touched the jug in the kitchen) but if you're worried about static you can buy special anti-static bracelets here, these clip onto the PC case your working on and are quite handy if you have to leave and come back thanks to the little dome clip on the wrist.
OK, so let's actually do an upgrade so you see exactly what's involved!
Finding the right RAM for your iMac is a simple process thanks to Apple providing all the info - to see this information click the menu top left, then select "About this Mac" and click the 'memory' tab - it will show you want RAM is currently fitted and even provide a clickable link to a guide like this one (theirs has drawn graphics rather than photos) with all the models listed so you can find the ram type and maximum RAM supported, without even opening it up - super helpful stuff from Apple! This is particularly helpful as the iMac doesn't use the kits we would typically expect to see in a desktop PC, instead, using the smaller SODIMM kits we would often be using within a laptop.
The iMac 27" we'll be using as our example comes with 8GB RAM out of the box, (two 4GB DDR4 SO-DIMM sticks) running at 2400MHz, and we'll be keeping this ram in the system while adding an addition 2x4GB sticks to the unused slots for a new total of 16GB.
The ram slots on the 27" iMac are hidden behind a little panel on the back, so we'll unplug the iMac and spin it around to see the access panel shown below.
Once you remove the power cable you'll be able to see a little button that was hidden just above it!
Pushing that little button just above the power input will cause the panel above to pop out from the bottom, giving us access to the slots.
You have to push it in pretty far though to really make it pop, and this might be tough (especially if you've got fingers like mine) but all we actually need is for it to pop out a little, then we can pull the cover off by giving it a quick pull - it doesn't need to come off straight, you can pull on the bottom and it will come out clean.
Once the panel is open you can remove the little cover and set it aside for now, you can see on the back of it are some instructions for exactly what we're about to do!
Now we can see the RAM inside, with little levers on either side which have white handles. Pushing these little white tabs outwards (like the little triangle arrows show) will make your ram slots pop to an easy to access position so we can insert the new sticks.
It can be tough to see the slots inside the iMac, but they do have little channels that will guide the new sticks into the slots so installation is pretty easy so long as you have you ram sticks facing the right way - with the iMac your ram should have the little indent in the gold bit on the righthand side when you insert it - pictures below to show the sticker side of our RAM kit is facing downwards.
Each time you insert a ram stick press it down until you hear the satisfying 'click' and it aligns with the pre-installed ram kits already inside.
Almost done! Now we push those little white tabbed levers again to have the ram slots tuck neatly back into the iMac.
Now we pop that cover back on, I found it easiest to go top side first and once it's partly in we just push on all the corners until it sits flush.
A great thing about upgrading RAM is you won't need to install any new drivers or programs to access it - you can view your new iMac ram total in the "About this Mac" option in the menu, then click on the 'memory' tab to see your new total - 16GB for me!
We hope this guide was helpful for those upgrading their own iMac at home, you can check out the Official Guide from Apple Support here for more pictures and be sure to keep your eyes on PB Tech Tips & News for more handy how-to guides!