I followed this guide to replace the HDD on my Mac Mini (2009, aka model A1283):

https://www.ifixit.com/Guide/Mac+mini+Model+A1283+Hard+Drive+Replacement/1063

The most tricky part is probably opening up the Mac. As I did not have a putty knife at hand, I used a rack cage nut mounting tool to create an opening,  and a very thin flathead screw driver to pry the case off, as explained in the guide.

EDIT: added picture of tool.