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Hi, I'm Mike G and I'll be assisting you. Could you upload a picture of the fixture?
With a pliers, turn the exposed nipple counter o clockwise to remove the entire nipple. When removed you can spin that plate off and reuse it after the lamp replacement. Trying to remove it with a tool in place may crack the globe.
The center post is a nipple and when loosened or removed that plate will thread off.
The close up pictures show that there are raised tabs that allow for the turning of the plate for removal. The stem goes all the way up and is threaded into the upper base of the fixture. Either the plate will turn off or the stem will exit the base, either will allow removal of the globe.
Nothing I could suggest other than an open jaw on a pair if pliers. It only has to move a little and then you can complete removal with your fingers.