You can try moistening the back a little, don't soak it, lay out on paper towel to absorb the moisture, place smooth plastic on top, piece of plywood on top of that, pile a bunch of books or other weights on top and leave it for a couple days. Any pattern in the material you place on top of the leather will transfer to the leather, so has to be smooth. If you are in an area with high humidity and warm that promotes mold growth, need to inspect after a few hours, air it out, and replace the weights and repeat.