It depends on the manufacturer of your particular inlet valve.
Most involve total replacement. Remove screws slide a knife behind the plastic face and cut away sealant. pull unit out and remove hose; install new unit to hose; re-goop and screw it back into place.
A temp fix can be made with a hose check valve screwed into the city water port.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW