That is a metal military valve. You remove the six screws and it should pull right out. You can replace with a metal or nylon (plastic) valve. These are available at most rafting stores. Here are some links:
Raftus is right on unless someone has stripped the screws and aquasealed/glued the valve in the hole. In that case some pliers on the lip of the valve and some wiggling should do the trick.
Some of the nylon valves have slightly different screw placement that doesnt line up well so I would stick with metal just to make it easy.
If you have a leak from the screws being loose or stripped...you can rotate your existing metal valve(after lifting it out) so as to drill new screw holes between the old ones. A little aqua Seal, or silicon, would help insure a seal. Of coarse, be careful not to drill too far. Just to go through a metal plat approx 1/8" - 1/4" down. I just did this procedure last week. Worked well.
If you have to replace the whole valve assembly, including the "boot"...that's a whole different story and could get very involved.
I'm assuming a lot here, but if by chance you want to replace it because it's leaking and you haven't already done so clean it really well with some canned air followed by a thorough wipe down with 303.
I agree with Incoop. Try cleaning it first. It still requires you to remove the screws and take the valve out of the boot. It is sometimes hard to take the valve out once you take the screws out. It may require some hard yanking. What kind of boat? Hopefully, the valve is not glued in. I'm in the Abq. area. Gimme a shout if you need a hand.
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