When removing the impeller, how do you hold the shaft still?

To remove the impeller you first take the front face plate off to expose the impeller. Then isolate the shaft from the motor with a pair of needle nose vise grips and using a pair of channel lock pliers grip the end of the impeller and rotate counter clockwise to un thread it from the shaft. If the shaft seal has been leaking there may be to much corrosion to turn the impeller in which case it needs to be broken off.

Take the screws off the outer cover fo the wet end. That will expose the impeller.
At the other end of the pump is a small metal cap in the middle. Pry that off with a screwdriver. That reveals the shaft of the motor which has a slot in it. This allows you to hold the shaft still with a large screwdriver.
While someone holds the shaft with a big screwdriver, you rotate the impeller counter clockwise to unscrew it off the motor shaft.
In my case the impeller was so caked onto the shaft with rust that we could not loosen it. We broke the impeller and still could not unscrew it. The screwdriver keeps jumping out of the slot in the shaft.
So the next step was to take the shaft out of the motor. On the far end of the motor are 4 small looking bolt heads. Undo those. They go right through the motor into the 4 legs of the wet end.
Stand the motor up with the wet end up. Pull hard to pull the wet end motor cover off the motor and you will then have the wet end and armature of the motor in your hands. Heavy.
Place some wood into a large vice and then grip the big silver part of the armature in the vice. Then unscrew the impeller.

You will need a 7/16 inch open ended wrench, a bearing puller, a phillips screwriver, a 12" peice of metal electrical conduit, and the bearings.
dismantle wet end of pump down to the impeller. Remove the shaft cap at the back of the motor(small circular cap in back). use 7/16" wrench to hold the shaft under the cap whiile spinning the impeller counter clockwise untill impeller is off. using a 1/4" hex driver remove the 4 through bolt located at the rear of the motor. seperate end caps on motor housing to access the bearings. use puller to remove. use the 3/4" conduit to tap new bearing into place. Reassemble unit.

Make sure you have an o-ring kit for this pump first

This can be very tricky at times. Quite often a sharp blow from a hammer on the wrench will work. Sometimes an impact wrench will work too. If that fails, then remove the fan cover from and try holding the motor shaft with channel locks, or vice grips. If it has a fan, you might have to remove it.

Are you holding the motor using the 'flats' on the shaft on the back side?

I have removed lots of impellers - usually I find the center has spun out of the impeller so I use channel locks to remove the hub. All the pumps I take care of are standard thread - might yours be 'backward'?

Try this, get a leather belt and wrap it around the impeller. About a foot from the end, put a stick under the belt. Hold the stick and belt together and try to lever it off that way.

Hi there ... I treed to explain what the process was on the motors I maintain for 4 different bodies of water ... evidently your motor is different

... If yours is that stuck, that you cant hold it with vice grips nor with a large flat blade screw driver (and there is no a flat you can find ont he shaft ... can you remove the back of the motor and grab the armature? There are probagly two or four 1/4 very long bolts that hold the front to the back.

How about if your put a heavy screw driver into the slot side ways???

There is that possibility that the manufacturer intends that you not repair it.

The impeller in my 4 HP, 2-speed Waterway Executive High Performance 56 pool & spa pump has a brass coupling embedded in the back side. This coupling can be unscrewed from the motor shaft by keeping the shaft from rotating while turning the impeller in the reverse direction. Then the seal can be tapped out.

Take off the back of the motor, put a wrench on the shaft and turn the impeller by hand to loosen. It may be reverse threaded.

That impeller may have been pressed on; good luck.