First of all take thge top off the machine, take the bottom off the machine.
Undo the screws on the front panel, you may find that only a couple are screws and the others are held by a spring clip, you can see when you look inside the top, take off the bobbin winder stop if it is on the front panel, put the stitch length dial on 0 and pull it off, undo the screw on the light switch and pull the knob off, you may need to undo the black nut on the switch as well, you can now take the front panel off.
you need a long screwdriver, put the screw driver through one of the slots so you can get to the screw on the gear, there may be two screws, slacken them off, once you have done that tilt the machine backwards, you can now see the drive belt that goes round a drive wheel directly in line with the shaft that goes to the top gear, in the middle of the drive belt there is a belt tensioner, slacken that off so you can pull the wheel and shaft down, you don't need to pull it all the way out just enough so the top gear pops off the top of the shaft, take the gear out and replace it and make sure the shaft goes all the way up before you tighten the screws, put everything back and tighten the belt tensioner, don't put the bottom on because you will now need to retime the machine.
email me and I will attach instructions for retiming, this site no longer allows email addresses to be shown so type in my address below as you say not as you read it but remind me of your model and problem.
Ken
drivewithken at blueyonder dot co dot uk, July 2009