1. First with a needle to the larger neckline to tighten up, the collar received the right size.
2. With an iron to repeatedly iron the neckline. Generally as long as not too serious repeated several times can be restored.
3. The seam on the line to tear down, otherwise there is no elasticity but do not wear.
If it has been particularly loose, such an approach would not allow the T-shirt neckline to return to its original condition. But you can make the neckline smaller, not too loose.