Here's an idea that I have used. Cut out the banner and replace it with another. But before you do that, get yourself a set of alphabet letters that will fit and fuse them on. Use a blanket stitch or pen stitch to finish. Then apply the banner. I did something similar to this and it worked out really well. You may want to hand stitch the new banner in place which is what I did using a blind stitch. If you think the wallhanging will not be washed, you don't need to sew it down.