Jan is right, we need more info. But the basics are:

* Increasing the diameter only increases the load on the motor, shortening the run time. On some boats it will increase the speed, on others it may just burn up the motor. Make changes in small steps, and bring the boat in after 90 seconds to check for excess heat in the motor and ESC.

* Increasing the pitch only will usually increase speed along with increasing motor heat. Again, make small changes in pitch to avoid overheating.

* Sometimes reducing the diameter while increasing the pitch will increase the boat's speed.

* Prop design can influence speed. On surface drive hydroplanes for example, changing to a lifting propeller can increase speed by lifting the transom out of the water, reducing drag, increasing speed, and sometimes reducing the load on the motor. However, some boats just will not work with lifting props, either cavitating badly or running with the bow too deep in the water. A club member once installed a lifting prop on a submerged drive Crackerbox, and it drove the bow completely underwater!

* If the boat is very free-running with minimum drag, increasing the load on the motor by increasing prop size will usually result in an increase in speed - up to a point. Past that point the boat slows down and the motor heats up. It is trial and error, hence the need to be specific about your setup in order for us to save you time.