How do you change water into water vapour?

By heating the water. Boiling it, which would turn it into steam (a gas).

The process of changing liquid water into a gas is called evaporation. Energy is added to the liquid water - generally in the form of heat. When enough energy is added, it allows the most energetic molecules at the surface of the liquid to break free of the liquid and enter the gas phase. The more energy that is added, the more water molecules will have enough energy to move into the gas phase. If enough energy is added, all the molecules will leave the liquid and from then on adding energy will just make the gas warmer.

Another way to change liquid water into gas is to drop the pressure above the liquid. In this case, it takes less energy for the molecules in the liquid to get away from the liquid so some of them will already be energetic enough to get away.