Examine the control panel of your microwave. There may be a specific "Lock" button or a button with a secondary use that controls the door lock. Look for a button with a padlock symbol.
Press the unlock button. You may have to hold the lock button down for a couple of seconds on some models of convection microwave. The microwave's digital display should give an indication of the door lock being disabled.
Pull on the door handle. The lock should be disabled and the door should open. You can activate the door lock again by closing the door and repeating Step 2.