[Alteration question] how to fix tightness across shoul Sewing Machines Parts and Sales in Los Angeles

I’m relatively new to sewing, and just finished the Meridian cardigan from Imagine Gnats (link: https://indiesew.com/products/imagine-gnats-the-meridian#product), only to find that it is a bit tight across the shoulders.

In an ideal world I would have altered the pattern beforehand to allow for my shoulders/chest being a size 4 while the rest of me is a size 2, but alas.

Is there a way to fix this? Any handy links I should be aware of?