Rather than replacing a single bearing that's inside the fan case, all 500 series models have a bearing and plate that are replaced together.  To get at it, you'll need to remove the outside bell housing and fan case.  Once the fan case is removed, you can then remove the fan.  You'll need an ice pick or something similar to insert through a small hole in the armature that's inside the motor housing.  That will put a bind on the armature so you can spin the fan off.  Once the fan is removed, you'll see the bearing plate that's held on by four screws.  Remove those screws and you can then remove bearing and plate.  Be sure to remove the motor brushes first, because the armature will usually come out with the bearing plate.