The manufacturer does not offer any support (from FAQs "19. Can I replace the battery in the headphone? No, you can not. The headphone uses a built-in non-detachable Li-Polymer battery that can not be repalced."), so you'll have to disassemble the headphones (apparently there is no video on YT, you could try ask for directions here, posting photos), find information about the battery, buy a replacement and replace the battery. Usually it requires a screwdriver set, prying/opening tools, a soldering iron (and consumables). Sometimes, glue is also needed