Remove the binding...you should still have a sandwich ( top, batting and backing) with a 1/4" seam allowance on both fabrics. Cut away the batting and add/sew flaps of fabric to the front and back using the seam allowance, obviously calculated to not only produce the new quilt size but taking into consideration any "extra" possibly needed to attach this enlarged quilt to a quilting frame. I then mitered all eight corners. I flipped the top back, carefully measured some additional batting that mirrored the new border, lightly sprayed the backing fabric with adhesive and carefully positioned the new batting onto all four edges. I then sprayed the top of the batting and again worked carefully to align the mitered corners. A few adjustment later and some careful pinning to prevent shifting... it was ready to go back to the long arm quilter along with the lengthened binding. I am pretty much a novice, learning as I go but with this quilt I learned two critical things: measure better ( not your mistake) and NEVER do this again...make a new quilt.