So , first you have to know how much RAM your iMac supports because this may differ from one model to another. for example the first models of iMac support only 2GB of RAM memory, that is 2 modules of 1 GB each. The late 2009iMacs support up to 8 GB of RAM memory, again two modules, but this time of 4 GB each. If you want to be certain about this and know for sure how much memory your iMac supports, you will have to read its user's manual that you certainly received when you bought it or have a look at the table you can find on this web page.

Once you know that, you will have to go to the next step and order the exact RAM that you need, the model that will be compatible with your iMac. You will find that information if you run a search with your iMac's name and model plus the amount of memory you want to buy and you will find the links to the online stores where you will be able to order it. After you get it home, you will have to know how to replace iMac RAM before you start disassembling your computer.

It's not that hard once you know where it is located. you will find the RAM slots on top of your screen. Before starting anything, unplug your iMac and remove all the cord and cables that connect it and it would be a good idea to wait for a few minutes before going forward. Then place your iMac screen face down on a soft piece of cloth in order to prevent its scratching or damaging. Then , on the right of the raise stand base you will see the RAM access door that will have to be removed with the help of a screwdriver. Then you will see some plastic clips that hold the memory modules inside. Take them aside as much as you can and then remove the modules that are inside and replace them with the new modules of a higher capacity. Put the lid back into place and that's all.<
If you did not completely understand what you have to do or how to replace the iMac RAM, here you will find a detailed tutorial with pictures and all the small steps that need to be done during the process.