From the Logitech support website:

1. Make sure that the time and date are shown on the LCD. If necessary, press (FN + F9) until the time and date are shown on LCD.

2. Press (FN + F10). You are now in 'Set mode' - you can exit this at any time by pressing the Esc key.

3. Set the correct hour by either typing the numbers on the numeric pad or using the cursor up and down keys, followed by Tab or Enter.

4. Set the correct minutes by either typing the numbers or using the cursor up and down keys, followed by Tab or Enter.

5. Choose either 12-hour clock or 24-hour clock by pressing the cursor up or down keys to switch between the options. Then press Tab or Enter.

6. Set the correct month either by typing the numbers or using the cursor up and down keys, followed by Tab or Enter.

7. Set the correct day either by typing the numbers or using the cursor up and down keys, followed by Tab or Enter.