Extend top edge of box

I want to add to the top of of a box.  The file is an STL.  I import, but I can't find a way to just extend the top edge of the box. I have holes and other tings on all 4 walls, so I can't just drag the whole box up it messes up the reset of the holes.  What is the best way to do this?


Thanks in advance.


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