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There are a couple of ways:-
1) Soak your dentures in hot water, but not so hot you cannot put your hands in the water. This will soften the gum & you should be able to pull the gum off.
2) Use an oil like olive oil or vegetable oil to soften the gum.
If this does not help, take you denture to your dentist, who will send it to his dental lab and they will remove it.
I hope this helps.
Good luck and my very best wishes,
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Dr. John BDS PhD