What can i do to refit/reshape a night brace for grinding teeth?

Nothing: I am not sure why you want to refit or reshape your appliance, but i would not recommend your doing it yourself. Go to your dentist to have it done professionally.

Night Guard: Not sure why you want to re-shape it. If it no longer fits over your teeth, there has probably been changes to your teeth or bite. Or, the guard has warped. Night guards for bruxism...Grinding or clenching...Are made to fit you teeth. They cannot be "re-fitted". If it no longer fits properly, a new guard needs to be made.

Better to remake: If you are referring to a generic night guard, throw it away as they have little value and may even do harm. For a dentist fabricated one, the time and money to try to re-adapt it is a bad idea. Have your dentist mae you a new one.

Whether OTC or professionally made, if it doesn't fit, don't wear it as it will actually cause harm. The generic OTC varieties are notorious for causing teeth to shift or bite to change. The ones made by a Dentist, if no longer fitting well because of damage, wear, or erratic compliance, probably need to be remade. Return to your Dentist or Orthodontist for evaluation.