Do you have a serger, a sewing machine, or a pair of really sharp scissors? If you have a serger then you buy more ribbing and attach with the serger to the new shape you want. I have only my sewing machine so I usually like to cut a new facing in a cute contrasting fabric. Use a shirt that you like the neckline of as your pattern and add seam allowances. If you have never seen neck line facing patterns you may want to look at a sewing pattern first to understand the construction principles. Since I use a woven fabric I make sure that opening will fit over my head before cut and attach it to the new shirt. Lately there are a lot of styles that have unfinished lines. This is the easiest way to do it. Just cut the neck out to the size and shape you want and let it curl. Be sure that you cut off less than you think you want as the unfinished edge will stretch. If you have this type of neck you may want to cut the sleeve and bottom hemlines off to match the style.