That is a fairly simple operation.
The throttle grip is held on by a single screw. Spring loaded so push in while unscrewing.
The throttle handle assembly is held onto the mount bracket usually with a nut on the inside of the handle and a screw. The Nut needs to be loosened and in some cases the screw has to be removed and the nut loosened again in order to remove the assembly.
Once the assembly is removed then the throttle shaft and gear is easily removed. Remember to grease everything all the way up the side of the engine while you are there and have the chance! Remove/clean/lubricate and re-assemble! Not all throttle shafts are the same so make sure you have the right one prior to doing the job.