I have recently replaced the ipellor on my 1996 Mariner 6hp. I hope the info is usefull.
1. Undo and remove the reverse gear tilt lock (on the exposed gear change rod between the leg and bracket).
2. Disconnect the gear change rod from the activation lever at it's upper most end.
3. Remove three bolts holding lower unit onto leg.
4. Gently withdraw lower unit from the leg. The lower unit will come away with the drive shaft, but the water tube will remain inside the leg.
5. Remove four bolts holding the impellor casing to the lower unit and remove.
6. Withdraw the impellor from the drive shaft and replace with new impellor. Remember to include the plastic washers above and below the impellor.
7. Replace the impellor housing onto the impellor ensuring that the vanes are sweeping the correct way.
8. The lower unit is now ready to be re-installed onto the leg. Gently guide the gear change rod up the leg and the water tube into the rubber sleeve on the impellor housing. Don't forget that the drive shaft must also be aligned correctly as well (three hands would be useful for this part!). It's actually easier than it sounds.