No sewing required, you just need a pair of fabric scissors.
Watch the video tutorial:
Step 1: Cut the neckline
Start off by prepping the neckline.
Decide how you'd like to cut the neck off - Follow the original seam so your cut will be even. Cut 1in wider than the original neckline for a roundneck. Cut 1.5-2in wider than original neckline for a wide, boatneck or off the shoulder neckline. Do not cut more than that because when you stretch the fabric it becomes much larger than you expect.