I am far from an expert modeler, but this is not a good model. Not only is it all tris for no reason at all, but every single face is overlapping at least one other face. It's also got six different materials on it (although maybe that is intentional?) and the UV map is bizarrely far off the UV space grid (which doesn't really hurt anything, but looks strange) and has quite a few overlaps in it as well.

If you are really intent on using this model for some reason, then I would remove doubles (when I downloaded your model and did this it removed 77 vertices! that's a ton in such a simple model), then convert tris to quads as RobynsVeil suggested above, then recalculate the normals and THEN just move the top vertices down like keppel said.

Honestly, if all you want is a simple T shape I would think it would be faster to just model one from scratch..

Here's a better view of it in Blender for anyone who didn't want to copy and paste the links above into their browser.