Samsung 840 EVO 1TB 2.5-inch SATA III for Late 2012 21.5 iMac?

You may want to rethink things here.

Apple uses a custom blade SSD in their systems which presently is only available from Apple. So your Samsung SSD will not work in the SSD port of the system. If you where thinking of swapping out the HD for the SSD the answer here is still a no without a special adapter. OWC had an adapter but has pulled it as people had problems with it when used with SSD's. So that rules out that direction.

OK, what are your options then...

First you could try to locate a real Apple SSD blade (used) if yours is able to support it. Thats the next rub here as some early versions of the 21.5" system don't have the SSD port present (review the IFIXIT teardown). The other option is to swap out the HD with a SSHD unit so you get the benefits of the deep SSD cache but still hold onto the storage space of the HD. As an example here the Seagate SSHD drive.

Frankly, I don't like any of these options if you have not opened one of these 'Thin series' before as they are a lot of work just to open. That leads me to the answer you won't like here (sorry).

Best to sell the system and get a new one with the configuration you want upfront. If you want the idea to upgrading things or just fixing things on your own. You will want to invest your moneys on a Mac Pro system. We now have 20 of these and boy! was it the right way to go. Yes they are expensive! but you have a much better system to start with and have the ability to service them as needed. BTW - We stopped buying the iMacs.