You cannot easily replace the power supply / battery unit ("BBU"). The ONT communicates with it to obtain status on power and battery. It's more than just supplying the correct voltage.
You have several options to deal with the low battery:
- Remove the current battery and press the battery silence button.
- Re-install the current battery and see if that turns off the warning beep.
- Buy a new battery. Online prices are very good.
- Earplugs.
Note that if you have a battery, the internet will stay up for a minute or two during power outages. If you want longer run time, you need to supply your own UPS.
Good Luck.