How do you replace reverse gear in 1974 vw bug will not back up?

Changing the reverse gear is not an easy task. Transmission needs to be removed,disassembled partly ( 50% ). I would highly suggest that you attempt adjusting the shift rod collar under the rear seat first. When reverse is engaged, the rod slides forward in the tunnel, pulling the collar forward as well.As time and miles add to the wearing out of all the related shifting parts, the collar may need a simple adjustment. Remove rear seat bottom, and the oval access cover at the rear of the tunnel.Put transmission in neutral. Loosen the lock bolt on the shift rod end of the collar and be sure to see that the shift rod does in fact slide further into the collar just a little more than it was when you first started ( about 1/8 inch more ) retighten lock bolt and test reverse ability. if it still fails you, it is time spend the $$ on a rebuilt trans. These can be as little as 350.00 or as expensive as 2000.00 depending on who you shop.

Good Luck with it!