To change the battery in Surface Pen, you open the pen, swap the old AAAA battery for a new one, and close it again. The way the pen opens, and which way the battery faces, depends on the pen model, and some models also have small coin cells in the cap. Once the pen is back together, you can confirm in Windows 11 that it still works.
- Open the pen, replace the AAAA battery with the + end facing the direction your pen type requires, then close it; Slim Pen and Slim Pen 2 have no replaceable battery.
- Pens with two side buttons also have two size 319 coin cells in the cap, and you may need to pair the pen with your Surface again afterwards.
- Check the pen afterwards in Settings → Bluetooth & devices → Pen & Windows Ink.