I haven't looked at that specific file in inventor, but I know how to change dimensions-
-go into sketch mode
-at this point you can either A) redraw that part of it to the right size, or if possible, B)
-click "general dimension" and then select the length that you want to change
-a little box with the dimension (38") should pop up, type in the new dimension (28")
-click ok or enter or whatever it is
-the length should be changed
*note* this might not work if other parts of the drawing are dependent on that dimension.. it will tell you its overcontrained then. I can't help you much if that happens, you probably will have to redo some stuff
I hope this helps you some!! hopefully someone else will notice and correct me if I missed a step
