Change the position of 3D box center point

I’m trying to center a character model onto a player object (a 3d box) but every time I set the model to the object box’s center position it sticks to the upper right corner of the box instead of centering it. I saw a tutorial a week ago that shows how to edit 3d points on the box but I can’t find it now. How do I edit the center point on the 3d box?