If it is not a rivet, it is a 'security' type screw to stop Tom, Dick or Harry from opening the case!
If this is the case (excuse the pun) you will need what is now commonly referred to as a 'Torx' style screwdriver (it isn't actually a 'Torx' screw but it is often referred to as 'Torx' because 'Torx' fits the description of screws that are not screws as we know them - if you know what I mean?).
Don't worry about feeling foolish about asking your local emporium about this type of screw-driver because they know diddly s&*t about them (but they might have one of them on their shelves all the same!).
From their original and limited, high-tech market, these screw types have become a cheap marketing ploy so search somewhere like 'Screwfix Direct' for a set of suitable tools to do the job you need (at a fraction of the price of your local 'specialist' tool supplier).