A way to change the aspect ratio while watching on tv

I would love a way to change the aspect ratio . tv shows are fine but for most movies I download I cannot watch them using servio as they fill the screen and therefore stretch it upwards. I therefore bought an external hard-rive which i plug into my bluray player this way i can watch them but i would prefer to use servio as it is handy to leave the hard-drive plugged into the pc.