Trim nail gun or crown stapler is how it was attached from the factory in all likelihood
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2016 North Point 377 RLBH, with a few mods, disc brakes, shocks, Sailun g rated tires, wet bolts
2014 Ford F350 Platinum sold
2017 Ford F350 Lariat, CC, 6.7PSD, DRW, Trailer Saver BD3 hitch
Assuming you do not own a nail gun or stapler I suggest you get some 18 gauge brads (nails) and use them to tack the door trim to the piece of molding next to the door. Brads come in different lengths so try 3/4 or 1 inch brads. Since they are so small I use a needle nose plyers to hold the brad while I drive it with a hammer. Once in place you will not see the brads.