You'll need to remove the chain/gear assembly from the back. Take everything off the front (auger bearings and gear box support) and pull the assembly out the front.
It's fairly straight forward.
Depending on the age, you might want to consider replacing the bearing behind the fan while you have it torn down this far.
In a similar style blower, I also added grease fittings to the fan. Helps to ensure the fan is free on the shaft should it encounter an obstacle requiring the shear bolt to break and spin freely.