This model appears to be a counter top model (if not, please reply back, since procedure differs).
You will first need to unplug the unit and remove the outer cover.
Then you'll be able to see inside where the door open button presses on the release lever, which lifts up the hook-like projections (latches) on the door so that the door will open.
Usually the part that breaks is the release lever.
You can find helpful exploded views for similar models (such as the GT4185)
here, but your best bet is to call the number at that site.
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