Stands for extension and C stands for compression.
Have a look at the linkage and determine if the it needs to be compressed or extended to change up the gearbox.
To change to the correct setting, simply press and release the button.
if both C and E are displayed, then the quickshifter is setup to cut on both directions. It is NOT advisable to use this setting unless it is really needed, such as for endurance racing where one rider rides with a race pattern and the other with a road pattern.
To change to the next setting (SENSITIVITY), press and hold the button until the LCD displays