Device Won't Power On (except when plugged into the wall/power source)

Hi-- I don't have any issues with the battery life itself. Just fine for the cycling I'm doing. I do, however, have an issue with the battery's retention of life. The device now only turns on if it is plugged into the wall. I was hoping there was a way I could easily replace the battery. Granted, it had been a while since I had used it (a few months because of a back injury). Can I replace the battery? How do I contact customer service to help me with this? Anybody else have a similar issue? Thanks!