How do I fix this purse?

Happens to me all the time. Just wiggle it back in place. Or you can take to a shoe repair and see if they can sew it in tighter for you, or if you want to chance it, you can try glue.

Been there done that, I just put it back into place. Like the idea of sewing it tighter, though, it shouldn't cost much or take long.

ive been trying to post this answer for you from the second you asked but it wouldn’t post so her you are. I took a photo of my response.

see my answer in photo. It would not post.

Shoe repair / shoe cobbler can sew it in place for a few dollars.

try using a channel wrench and closing the D ring tighter. Or sew the leather closer to the ring with a sewing machine.

Insert a metal key ring circle with a smaller circumference than the D ring through the end of the leather strap. Then hook the carabinier hook through it. You can keep the D ring in place; no need to remove it.

The straight part should be in the leather loop. Push it back where it belongs and add a blob of clear e-6000 on either side of the leather when it is in place. It dries clear like a silicone and will keep it from sliding back. If not enough add another layer of the e-6000.