the sight should on the bottom have an allen key they you can loosen to adjust it.
also you have to remove the m16 hand guard to access the allen key at the base of the barrel to adjust that as well.
then you can turn both and align to where u want them.
then just tighten them down.
mine was the same way after adjusting them and tightening them it hasn't moved since.

edit: i don't think u need to adjust the m16 handguard to adjust the sight. i did just so it was lined up with the sight