Assuming the main body of the valve is ok you can change all the gaskets and everything on the outside without having to cut huge tubes.
Take the cap off and the then you should be able to unscrew the front part of the valve. Hold onto the back of the valve body by holding the back of it from the other side of the deflated tube. You may need a big adjustable spanner to loosen it.
Once undone remove the old gaskets etc. At this point you can't let go of the valve body or you'll loose it into the bottom of the tube and it's a bugger to find it again so make sure you have everything ready and within arms reach before you get to this point!
The rubber valve is usually what perishes and is screwed to the plastic stem that you push in to deflate. You'll need to turn the valve body around (careful you don't drop it!) and unscrew the rubber valve, probably with the help of pliers to get it undone. If it won't budge be careful you don't damage the main housing as if you break that it means cutting the tube and patching to replace it.
Replace the plastic stem and spring and then screw on the new valve from the back making sure its tight.
Then just fit it all back together with new gaskets and new cap etc if you want it to look shiny as well as be functional.
A bit fiddly when you do the first one and will probably involve some swearing but you'll soon learn and the third one will be a piece of cake!
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Freelance skipper, RYA Powerboat Trainer and Advanced Examiner