How can I repair this garage door trim and weather stripping?
Is the weather seal screwed in? 

I would recommend removing the weather seal and replacing it with new.

Install the top piece first. Do not cut it long so you have to force it into place; cut it to fit tight at the ends but you should not have to force it into place. use ring shanked colored nails and nail about every 6-9". Do not over tighten the nails, just bring them flush to the weather seal. Cut the sides to length, again, not so tight that you have to force it. Square cut all ends, do not attempt to miter this product. the rubber flap should form a U to the door, not a 90 degree. Clean it all up then use a good caulk along the edge of the weather seal and the siding.

Done like this you will get most of that bowing and gapping out of what you have now.

Done right, this will be a huge improvement. Take your time and get the weather seal cut to the right length and nail carefully and often.