What you need is a voltage regulator. Normally, finding a regulator for 5v is simple, since 5v is a very common voltage for electronics; however, most of them will want at least 7.5v or so input voltage. You could pair up your batteries to make a 12v input, but with a linear regulator, that would be wastefull, so I would suggest you get a switching regulator with a 5v output and setup your batteries in parallel groups of two in series. Also, make sure that the regulator's output amperage matches or exceeds that of the supply you're replacing.