Welcome to the forum.
Usually this is due to wear in the control arm causing sloppy shifting or wear on the drive side thrust washer inside the transmission.
There are a lot of variations so please post the actual model number from the ID tag under the seat .'Should be 13???????? or 14?????????
Depending if it is a lawn tractor or garden tractor.
Once we have the actual model number we can find the parts list so every one know what we are all talking about.
IT s a very simple system for changing FNR just a few too many variations to be more specific at this point in time