Check out any of the mods in this section.
Steering motor replacement is pretty self-explanatory.
1) remove PCB cover
2) pull out original motor carefully by the wires
3) remove gear from original motor
4) cut wires to original motor
5) install gear on new motor
6) solder wires onto new motor
7) install new motor and test
8) If steering is reversed, reverse wire connections to the new steering motor
9) if new motor is longer than the original, make a hole in the PCB cover to accommodate it.
10) re-install PCB cover
11) Profit! (just kidding) you should be done now.
If you need more detailed instructions on the particulars of this process or the terms used it might be a good idea to Google for tutorials on the subjects or practice soldering etc on something else first.