Model year for iMac

It's a late 2012 iMac.

You'll notice it's considered Obsolete by Apple. Also that's the year they started gluing the screen to the computer case instead of holding it in place with strong magnets. That makes it extremely difficult to get the screen off without damage. Apple won't work on it but an Authorized Apple Service Provider  might. It's iffy. They will want to get the drive themselves so won't use any parts you provide.

You've gotten a good 10 years service out of it and I recommend you replace it with a new iMac, one that will be 3-4 times faster that the one you got and will run the newer systems. Of course you will have to replace any 32 bit apps with 64 bit replacements or alternative apps. To see which of your apps will have to be replaced download and run the free GO64 app.

If you go ahead with replacing it yourself contact Customer Support at OWC (MacSales.com) to get the correct SSD and any needed cables, etc. for the installation. Also ask about gluing the screen.

Don’t order any “parts” for your Mac. Any you find online are used and likely counterfeit. Plus, there are no user replaceable/upgradeable components other than the RAM.

Unfortunately, you have an older iMac that Apple won’t service. I would strongly suggest you consider replacement.

Everything has being said. I once replaced a working rotational hdd with a more efficient and faster ssd on a similar machine. Beside the ssd has been attached with scotch tape which sounds stupid but is enough, the removal of the sealed display has been a nightmare also for me that I've replaced tens of hdds with ssds on 2009/2013 iMacs. So: if you can't afford a new iMac and the actual performance is enough, be careful. Let a specialized center do it for you or somebody that has good experience on such operations considering also that the display removal implies the detachment snd reattachment of power and data (displayport) cables on the MB. You must be very delicate need a guide (ifixit.com). THEN: surely you'll have a temp sensor on the actual hd which will not be present on the ssd. This will probably bring fans to alwsys run at max speed. You can solve with Macs Fan Control.

Concluding: not impossible but a very delicate operation and a lot of patience. And if the display goes broken... almost impossible to getva replacement (if worthwhile...).

See below.