Here's an example of a small, self-contained 5 VDC switching regulator that could handle any voltage from a realistic battery pack (and many common small solar arrays as well). Theoretically, one could replace the on-board linear regulator with it, but given the level of technical prowess (or lack thereof 
) JonnyA admitted to I wouldn't recommend it in this case. Instead wire up the output to the 5V pin. You can do this since this is a regulated power supply. Don't do this with an unregulated supply, like batteries, even if the nominal voltage is around 5 VDC.