How do I resize a folded box without messing up the folds on the top or stretching out the squares (thus messing up mapping)? I'm using a classic folded box that has already been created by someone else.
I want to increase the height, but when I scale along the y axis, the folds on the top get stretched too much. 

How do I just stretch the bottom of the box in height and leave the folds alone?

Edited for new issue: When I drag and stretch, all those little squares get messed up and then when I map a graphic onto it, it looks all stretched in that area.
There are many ways, a simple one is:


go in edit mode (tab)
select all edges in the base of the cube (xray mode can help you to select all the base)
G (to move the selection), Z (to block the movement only in the Z axis), move mouse to decide how far to move the base of the cube




HOW TO FIX THE UV:


fix the geometry in edit mode --> the texture is now stretched





go into UV editor, select the faces with stretched texture, go into UV editor, select the base nodes (the one that you moved to change the size of the box) and move them into the UV editor to fix the textures: