There's an exploded diagram of the machine, will give you an idea how it comes apart.
The impeller is retained by a single nut. Be careful not to damage the impeller when removing the nut (if you haven't removed it already). The nut will remove in the direction of rotation of the engine... By the looks of it, it's right-hand (normal) thread.
If the nut is very tight, remove the sparkplug and stuff about as much rope into the plug hole as you can. This will lock the engine without doing damage, and allow you to reef onthe nut.
Once the nut's off, the impeller is "keyed" to the shaft by having a flat spot on the shaft and corresponding one ont he impeller. This means it won't turn on the shaft more than a couple degrees. Use penetrating lube and gently rock the impeller back and forth while pulling and it should come loose. if you do damage the impeller, it's impossibly cheap (under $4!).