The large button is located on the side of the device right next to the USB port and the charging LED.
The large button can be used to turn on the device. The button allows for custom interactions during operation such as: turning off the alarm, sending SOS, etc.
The reset button is recessed and is located right next to the USB port in the middle.
You will need a paperclip or another pointed object to press it. It restarts the device or resets it to factory settings. More information on resetting.
The USB port on the side of the device is used for charging. Please use only the supplied power adapter and charging cable for this purpose.