Where the sleeve links the collar, that is, the end of the collarbone, sew it with needle and thread from the back and close it to your satisfaction. This will not affect the appearance too much.

Needle (sweater needle) can be worn with strong thread or thin elastic band, and can be worn along the collar, and the size can be adjusted and fixed.

You can sew the back neckline of the clothes in and make minor changes, but you will sew icons at the seams that will not make the clothes look weird. It depends on people. I don't know how to put a collar on it if there is a big change. But be smart, or you will ruin your clothes.