I realize OP asked about upgrading one segment. If you want to upgrade in one direction of your trip (e.g., return) you can generally select, for example, the a main cabin seat for your outbound and the a first seat for your return at the time of booking (though it would apply to all flights in that direction). Otherwise, if it is one segment in one direction of your travel, I would do what other suggested and select from the seat map after booking.