How do you replace the battery?

How do you replace the battery? I have had my flex for about 19 months. I have maybe used it a total of 6 months off and on. Now, the tracker will not hold a charge at all. I called the company. They said that they do not currently repair devices and that they could not replace it because it is out of warranty (fair enough). They offered me 25% off of another device. However, before I fork over another $75-$150 I was hoping that there might be a way to salvage the one I already have. Any ideas??