Hello!
These are opposite alterations. Easiest is to lower the back. In this case you reshape the shoulder seams as well. The collar will not be shortened.
Squaring the shoulders is a large alteration, because the sleeves have to be removed and widened, if necessary.
So much for the basics.
If you have an erect posture the body is usually longer in the front than in the back . Other way round with a stooping figure. This is what a tailor calls the "balance". This would have to be a pattern for the erect posture(if it's only square shoulders, the seams would be raised by the necessary amount):
https://img267.imageshack.us/my.php?image=erect2oo6.jpg
Back to your problem. If you have square shoulders and the coat has not, there should be a "gap" around your neck, because the coat/collar is lifted by your shoulders. If that is the case, squaring the shoulders will be best(or try a different coat-maker).
If you have an erect figure(see picture) and the front part is not pulled up too much (front will be shorter than the back), you can have the back shortened.
In case you have strong blades, easiest alteration would be to shape the coat's back in that area with the steam iron, given that the fabric allows it(flannel or any softer material is best). When the cloth is stiff, a dart might be necessary.
In either way I would recommend to look for a coat-maker, who cuts a squarer shoulder.
Hope I could help. I know all the correct terms in German, so this explanation might sound a bit weird. My apologies for that.
SG