Whats wrong with my htc evo 4g?

whenever i try to do a system update on my phone, it will restart (like when you first get it) then it will show the little android guy standing next to a box, with an arrow pointing from the box to him. then it starts to download, then an error sign pops up (its a triangle with explanation point in it) and stops the download. does anyone know whats wrong, and why my phone wont update?