Depending on what model crank you have it's possible that the chainrings are riveted together and can't be separated. Otherwise, chainrings that are bolted in place usually require a 5 mm Allen key on the bolt side and, sometimes, a spanner to keep the nut from turning. If you can remove the chainring or substitute a double crank, you will have to reset the front derailleur's low limit screw to keep the fd from shifting to a nonexistent ring.