Checking the manual first is always a good start.
I first went to this, on page 3-76, which redirects setting it up back up to page 3-41 if the vehicle is equipped with the color LCD.
The operational/non-operational setting of the one-touch lane changer can be changed by a SUBARU dealer. Contact the nearest SUBARU dealer for details. For models with the combination meter display (color LCD), the setting can also be changed by operating the combination meter display (color LCD). For details, refer to “Vehicle Setting” F3-41.
Unlike the page indicated, it's actually 3-40 to 3-42 that pertain to the one-touch lane changer settings. Check there, if your vehicle is equipped with the color LCD screen.