its a long time ago someone asked. Your tractor probably hasnt done a lot of hours because 95% are already modified by the dealer, which is a permanent cure.
Onder the center section of the cab floor (tunnel) is a row of bolts, right before the hi/lo and pto levers
If you undo the 2nd from the right, you see a sliding bar (to which the shift fork is attached) and with a screwdriver you should be able to poke the shift fork back to neutral, on the notches in this sliding rod.
If you cant get it done, undo the housing of the shift lever with four M8 bolts, then you'll get the picture of how the system works.
There is a modification kit for these tractors, a weld- on thing that you weld on top of the transmission housing, under the gearlever cover. Then it will never happen again.
But mostly once it starts doing it, you will have to pry it back to neutral more often because of wear. This modification kit is quite cheap actually, but installing it is some work because yo have to take the cab floor out and the transmission lid off (when a dealer employee was doing our 5245 years ago, by welding above the small hole of where the gearlever column is mounted, the oil vapours suddenly ignited and this flash burnt off all his hair. When he returned from the barber to shave the back of his head too, they took off the entire gearbox lid to work safe.