back of the shift button housing is 2 or 3 phillips head screws. from there you can pull it apart, on some ES machines, you also have to remove the left hand brake lever to get them off the handlebar. you'll just have to dig in to it to see.
also, how is the condition of the battery? I would make sure it is positively charged (holding around 12.8 volts at rest)
a discharged battery causes all kinds of issues with these ES machines. from there, if that all checks out you may need to dig in to your connectors for the shift motor/angle sensor/shift buttons to make sure nothing is corroded or loose.