just remove the keycaps and clean it by compressed air
you
don't want to do this blindly with mechanical keyboards. not all switch mechanisms are happy with having compressed air shot into them. please don't actually do this. of course, this is a perfectly reasonable method to deal with rubber domes, but this isn't a rubber dome keyboard forum, so instead of cleaning your RD board you should just toss it out and buy a reasonable keyboard.
if you blast with a compressor or compressed air, leave the caps on. and shoot the air perpendicular to keyswitch openings.
honestly i find pulling the keycaps off and just using a stiff brush and a soft cloth takes care of 99% of gunk.