If it's the same as the Vee mudguard there are square metal threaded catches on the inside of the mudguard for the bolt to thread into. Take the wheel off & now that it's spinning, look up under the guard to where it is located, spray with WD40 or similar release oil, hold the metal catch with pliers & unscrew bolt.
Best to take wheel off first when initially removing mudguard, spray all 4 fasteners/bolts with WD/release oil, Tap bolts with hammer then unscrew......As stated in a previous thread, the spinning metal threaded catch may damage your plastic mudguard if not careful.
Anti seize grease when re-assembling.....
The two forward dome/mushroom bolts (not brake line bolts) have collars on them to fit into mudguard.....