How to increase RAM memory on iMac 21,5 inch, late 2012?

You'd have to check the info specific to your model here: Install memory in an iMac - Apple Support. I've upgraded memory in various Macs over the years and it's usually rather easy. There are YouTube videos on the topic as well.

Similarly, switching to an SSD would definitely help, and they're very cheap these days. I prefer buying from Newegg as their reviews are accurate (unlike Amazon), their sources are good, and their service is excellent in my experience. You may run into limitations with your SSD speed because of the Mac's I/O. SSDs are handy to have for backups anyway - just buy an internal and a case for it, so that if you don't see a performance boost, you can use the drive for other, external purposes. It's also often cheaper than buying an external drive outright.

Web search turned up a number of articles on boosting performance on old Macs. I don't think we can link to external sites here.

Have fun - it's entirely worth it to try. And if it's not enough, you can always repurpose it as a media server or something else (again, quite a few external sites have info on using old Macs)

3Memory is not removable by users on iMac (21.5-inch, Late 2012), iMac (21.5-inch, Late 2013), iMac (21.5-inch, Mid 2014), iMac (21.5-inch, 2017), iMac (Retina 4K, 21.5-inch, 2017), and iMac (Retina 4K, 21.5-inch, 2019). If the memory in one of these computers needs repair service, contact an Apple Retail Store or Apple Authorized Service Provider. If you’d like to upgrade the memory in one of these models, an Apple Authorized Service Provider can help. Before you schedule an appointment, confirm that the specific Apple Authorized Service Provider offers memory upgrade services.