Assuming it was installed properly, there will be a large plastic nut on the back side that you back off. Once you disconnect the hose, the fitting will slide through the hole into the livewell. Should be a standard size and any marine place or mail order will have it. It may be tough with the sealant, but it will come loose. Not gonna be easy as back side access is usually tight. Might need a second set of hands to hold the inside if it starts to spin.
As far as a sealant to replace it goes - Use 3M 4200 or Boat Works Life Caulk. Do not use the 5200 or anything designed to not be removed at some point for exactly the reason you're having to replace the fitting.
Wannabe...