Doors – How to disassemble a simple door knob

You need to remove the key set section as pictured on the left. It may be very corroded and require a bit of persuasion to split the halves and remove from the deadbolt assembly. Once you have removed the keyway from both sides, there should be nothing but a couple of screws on the striker/jam plate holding the bolt assembly.

I'd drill the existing screw head off, use a bit slightly larger than the shaft of the screw but smaller than the hole in the knob. The stripped out screw should keep the bit fairly centered even when used at a slight angle. Once each head is removed you can pull the knob and try a screw extractor on what's left, but there's a good chance you'll need to replace the entire lockset at this point.