Originally Posted By 220:
You'll need the correct spanner wrench and something to support the lower (you can put the lower in a chair and sit on it to secure it). Then simply loosen with the spanner wrench. It does take some force to break the staking, but that is how it's done.
this, you could also get a small screwdriver or scratch awl and try to remove some of the metal. I've used that method on one of the more stubborn colt castle nuts i've removed.