Hi Natalie,
Square it up by blocking it! It is not out so much so blocking it (I think that is the correct term) is a real option.
There are tutorials on how to do this, but the basic are that you lay the quilt on a clean sheet on the floor, dampen the quilt and and manouver it until it measures the same all the way down and accross and then leave it to dry!
Hope this helps
Hugs
Caroline