The phrase "to replace the transmission" means to remove the damaged or malfunctioning part of a vehicle called the transmission and put in a new or working one.
Sentences with «to replace the transmission»
To replace the transmission fluid on this vehicle, remove the drain plug and drain the fluid. (yourmechanic.com)
I previously replaced the transmission with a used one and had the rear seals replaced, as well as the electrical connection from the terminal switch near the transmission. (articles.orlandosentinel.com)
I had to completely replace the transmission and within 2 weeks had to take the car back into the shop because the transmission was not working right again. (autolist.com)