The battery is charged by plugging the DC plug into the receiver located under the seat. Once it is attached, you can connect the other end of the plug into an AC wall outlet.
If you can not place the bike next to a wall outlet, you can disconnect the battery connector at the seat post's bottom end. Twist off the adapter until it detaches, then open the seat post's quick-release lever and slide out the seat post. Then you can take it with you to charge as described above.
After charging the battery, you can slide the seat post back on the bike, use the quick release to adjust the seat to the desired height, and close the quick-release lever. Then reattach the twist connector to the bottom of the seat post and secure it tightly.