Make sure whatever you do don't strip them. If they are too big for the hole. Get a regular drill and use a drill bit just a hair smaller than the shaft of the screw to make it bigger.
They're M3 sized...standard size used for radiators, you can drill them out, or go to any hardware store and pick-up some screws, 30mm long would do for most fans
Your case should have come with long thinner machine screws for that thay are intended to go completely threw the fan and into the case and are also ment for radiators not the fat large bore threaded screw that come with fans but if u can get to the fan holes from the inside then just drill them out but make shur u are screwing those fat screws into the plastic threw the metal and not the other way around as this will just strip out the metal and not hold anything