Logitech G603 LIGHTSPEED Frequently Asked Questions Manual page 8

You can connect the G603 gaming mouse to your computer, phone or tablet with the included
LIGHTSPEED USB receiver or using Bluetooth. Before you connect your mouse, make sure
you've installed one or two AA batteries.

Installing the batteries

USB (LIGHTSPEED) mode

Bluetooth mode

Switching between USB and Bluetooth modes
Installing the batteries
1. Position your thumb below the DPI Cycling button as shown.
2. Use your index finger to lift the keyplate, leaving the battery cage exposed.