how to unstretch a vest?
 

I bought a long-sleeve Mexx sweater a few years ago, and it's been stretched out to a point where it's loose on me. I am wondering if there is a way to safely unstretch it.


I'm not sure what the material is called, but it's those typical formal, diamond-patterned sweaters that people wear over a dress shirt or just as it is.



TIA