Solution

The air above the flame is hot. If a glass is placed on top of the candle the air inside is heated up. The candle shortly after extinguishes, due to the lack of oxygen. When the air inside of the glass cools down, the air pressure decreases.

If the water could not enter the glass, the air pressure inside of the glass would be lower than outside of the glass. The arrows in the picture below illustrate that case.

The pressure inside of the glass decreases. The water is pressed into the glass and the water level inside of the glass rises due to the outside atmospherical pressure. The room for air in the glass grows smaller and the pressure rises. The pressure inside of the glass rises as long as it is equal to the air pressure outside again.