I would go to your local dealer parts department and order a new one. Being a part that would be replaced in an accident they may even have one on hand. They would be best to tell you how. Probably just some clips.
Figured out how to do it. There is a rubber plug halfway on the inner door side which you take off and find a phillips head screw. You remove this screw carefully because it can fall in the inner door cavity. Then slide the molding backwards toward the rear of the door. The molding comes off fairly easily. I found the broken clips( which by the way are very flimsy) and used epoxy to re-glue them on. Once dry, I refit the door lower molding and re installed the middle screw. I recommend putting a little vaseline or similar substance inside the clip that retains the screw as the screw very easily falls out. Once done,the molding is repaired and wow- no cost!
Is this the trim on all 4 doors at the bottom? I'm asking because on close inspection of my 2014 at the rear end of each peice of trim I can see a sticky pad that has come half unstuck from the trim, and the way our winters are in Canada I can see that trim breaking off if hit hard enough.
I want to either reattach or have it replaced, it's still under warranty.