Being taller than your opponent has many advantages but there are some aspects of boxing that play into the hands of the shorter boxer, if they can be exploited.
First off, the fighting style might be hard to adjust to, in particular with timing counter punches, as well as the distances and angle of the punching involved.
As the taller fighter will likely have greater reach, and would utilise this by keeping the fight on the outside, the shorter fighter should try to close the distance and fight on the inside, avoid clinches and use pressure. During inside fighting, taller fighters would also need to be aware that any collisions of the heads might do more damage to them than their opponent, due to the point at which the heads meet; the forehead, or top of the head of the shorter person landing on the mouth, nose or eyes of the taller boxer.
If indeed the shorter person has shorter arm reach, this could lead to having faster hand speed, due to the smaller distance their arms need to move to extend.
Taller boxers in this situation should use exploit their longer range, and clinch when the action is heavy inside.
Boxing Unboxed 