Here’s the user manual on it but I don’t see a battery replacement:
See id this helps:
You may adjust the ringer, speaker, handset and headset volumes independently with
the VOLUME ^ and VOLUME v buttons. Your telephone comes with predetermined
settings which can be adjusted to individual taste. You will hear an error tone when
the maximum or minimum volume level is reached. During volume adjustment, a
"dashed line" icon shows in the display.
Press the VOLUME ^ or v button while in on-hook mode.
NOTE: To turn the ringer off, on, or change the ringing pattern, see
Selecting A Ringer Tone.
Adjusting the Handset Volume
Press the VOLUME ^ or v button while the handset is in conversation mode. You may
choose from four volume settings: LOW, MID1, MID2, and HIGH.
Adjusting the Speakerphone Volume
Press the VOLUME ^ or v button while in speakerphone mode (SPEAKER indicator is
on). You may choose from four volume settings: LOW, MID1, MID2, and HIGH