1) Remove ear loops or headbands from your CS50/CS55 headset.
2) Locate the two sets of grooves on the sides of the headset. Squeeze them both and pull outward to remove the battery door. (Occasionally, these battery doors stick. You may need to pull or wiggle with considerable force.) - see picture below
3) Beneath the battery door, you will see a rectangular battery attached to the headset with two white plastic connecters. Slide these connectors apart to detach the battery.
4) If you have a replacement battery, attach it by clicking the two white plastic connectors together. Then reassemble the headset by replacing the battery door. Reattach any ear loops or headbands and charge for at least 2 hours before initial use.