You *may* need to remove the bolt catch first, like the M4 it depends how far the two parts were screwed together. Punch out the pin, remove the bolt stop, then push in the mag release button until the leaver clears the receiver; start to turn that anti-clockwise and the two parts will unscrew from each other. There is a spring between them as well.
The bolt catch is held in by a single roll pin. It's a bit tricky to remove due to its proximity to the receiver; as in it is hard to get a round punch aligned to the pin without scaring the upper. It is possible, just difficult. Brownells (among others) sells a special punch that has a flat side; you want to punch the pin towards the muzzle end, not the other way. Keep in mind there is yet another spring under the bolt catch. It is also a good idea to protect the upper with some tape or whatever so you don't end up scratching it. Good luck!
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