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The Late 2012 21'5-inch iMac has got non-replaceable RAM. You can replace it but it's very difficult because you have to unmount the whole Mac, and you will void the warranty doing this.
Anyway, if you still want to upgrade memory, you can buy more RAM at OWC > http://eshop.macsales.com/shop/memory/iMac/2012_21.5/DDR3L You have the steps to upgrade RAM in this site > http://blog.macsales.com/15932-what-does-it-take-to-upgrade-a-2012-imac-21-5
Do this at your own risk. If you do something wrong, Apple may refuse to repair your Mac
You cannot upgrade RAM in the 2012 or 2013 21.5" iMacs.
If you are encouraged to upgrade RAM on your own, you should be aware that once you open the computer to modify it in any way you will void your warranty, and Apple will no longer support the computer for you for any future repairs. I urge you not to be enticed down the DIY path.
If you need the new 21.5 inch iMac with the 16 GBs of RAM, you could always sell this iMac and use the money to purchase the 16 Gb RAM Model.
You, initially, had the first 14 days of use to evaluate this iMac and return unconditionally for a full refund.
It sounds as if you have had this iMac for longer than this.
And Kappy is correct.
You open this up to add the extra RAM yourself, you not only void the free, complimentary one year of AppleCare warranty service, but you void the chance of purchasing an extending the warranty service for the next 3 years.
I strongly advise NOT trying to upgrade this yourself and try to sell it and get a more powerful iMac model.
Adding more RAM is not going to speed up your Mac. Video editing demands use of the CPU and GPU which means VRAM. Nevertheless, the hardware you have should be more than adequate to the chore even with only 8 GBs of RAM.
Adding RAM only makes it possible to run more programs concurrently. It doesn't speed up the computer nor make games run faster. What it can do is prevent the system from having to use disk-based VM when it runs out of RAM because you are trying to run too many applications concurrently or using applications that are extremely RAM dependent. It will improve the performance of applications that run mostly in RAM or when loading programs.
If you have specific hardware needs, then you should evaluate those needs before you purchase a computer, not afterwards. That way you buy a computer suited to your needs instead of worrying about whether the computer is suitable.