When you power on the J3, the stock Cowon main menu appears. In the lower-right corner is the "power on/off" button.
If you drag that power button to the left, it will reveal three icons. Each icon corresponds to the main menu formats 1, 2, and 3. Tap on whichever format you want to see, and that becomes your main menu choice until you repeat the process and change to another main menu format.
Lynx itself has a slightly different way of getting to the main menu selection popup, but the above screenshot (which is taken from the PDF user manual for the J3, available from the Cowon site
) is how the stock Cowon main menu selection works.