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Put in a ziplock, and submerge in simmering water until warm and tender - somebody else will have to chime in on how long to expect. I use a Sous vide snd submerge for an hour or 2 at 165 for reference.
I vacuum seal any leftover beisket and then go the sous vide route when I reheat it. I haven't done it for a while, so I'm not sure of how long I leave it in the bath.
If your doing it for a family dinner or for company then the above posts are good recommendations. If it's just yourself then I reheat in a microwave on low. Just till warm.