This is possible in a 27" model, but not in a 21.5", or only with a lot of effort. Moreover, it is hardly worth it, since the latest operating system no longer works there either.
If you can't do it yourself, then don't bother and buy a slightly newer, possibly used one.
But if you unscrew it, then put a fast SSD disk in it right away!

I installed it in the 2012 model without any problems.
Greetings

I installed it in an Imac 2013 without any problems

Yes!