Look for a small set screw at the base of the handle.
How to remove screen door lock barrel.
Learn how to replace or change the security door lock under three minutes.
Remove this screw using a drill or screwdriver.
Did you lose the key somewhere near your house and now you are worrying that perso.
Then you can take the barrel to hardware store locksmith and get it keyed the same as your other doors if the base key shape is the same.
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Grasp the interior trim in one hand and the exterior trim in the other and try to pull them away from the door.