How warped is it? If it is severely warped, you need to take the warped part out and resew the pieces together. If it is only a little bit warped, taking a extra deeper or shallower seam in the warped area should do it. If it is only slightly warped, pressing might help. If you want to hammer it into place, you would likely want to get the special mallet made for using on fabrics. I can't remember what the name of the tool is, but it came out a few years ago especially for making intersections flatter where several seams came together. If you do not want to purchase the special fabric mallet, you would be better off purchasing a regular rubber headed mallet from the tool section, rather than using a hammer. The metal head of a hammer would be a bit hard on your fabric.