There are a couple youtube videos that show the process. I just changed mine to the brass fitting. The video will show how to extract the old fitting. I also put the new fitting in the freezer and the installation was much easier. Here is the link I used.
dc, if you mean your carb is leaking, that may just be the needle valve in the float chamber. It may be old and worn, or has picked up some muck from the tank. I suggest you drain and clean the tank, to remove any accumulated muck out from inside it, by removing the gas tap. Remove and clean the carb, especially the supply pipe and float chamber. If your carb hasn't been serviced for a long time you can get a service kit, which comes with new gaskets and rubber bits, etc.