What model/year is it?

I've successfully changed out the HDD in my 24" iMac with an SSD. However, these newer iMac models are increasingly difficult to open up and tinker with (for good reason)

You can see for yourself the steps involved in swapping out the HDD here: https://www.ifixit.com/Guide/iMac+Intel+27-Inch+Retina+5K+Display+Hard+Drive+Rep...

As you can see, they use a lot of specialist tools which (I guess) only a few specialist shops would have. And you run the risk of damaging components without the assistance of professional/capable help.

Additionally, if you're swapping out the HDD to put in an SSD, you'll need a caddy/converter like this: https://www.amazon.com/gp/product/B002B4HHZ4/ref=oh_aui_detailpage_o02_s00?ie=UT...

MacBooks and iMacs were very easy to tinker with up until their retina series. Now it becomes a serious consideration of if the effort is worthwhile.

My advice would be to find a reputable, professional service.

Although I absolutely abhor the newer Apple machines and how un-serviceable they are (also applies to other manufacturers that have gone the same way with some of their products), your best bet in DB is P-Solutions. Give them a call (2987 1777) if you don't want to carry the machine to the plaza. They do home servicing for a small fee afaik.