As others have said, it may be best to have it hired out.

On a side,
I replaced two panels of a 14' X 14' commercial door myself a few years ago.
I consider myself a tad more than handy with tools so it did not seem like a
big deal. In the end it was not a problem, but the cautions of the job being
a bit dangerous are true. Standing on an extension ladder,12 feet in the
air, unwinding the springs was an adventure!
For me, the funny part was when I was looking for the replacement panels, a
garage door company told me that I could not do it myself! Comments like
that just make me more determined!
I ended up finding two used panels from someone that drove a truck through
their overhead door.
Greg