Under normal conditions, how could you reduce air pressure?
In an air-tight container, permit some of the water to drain away without letting air back in to take its place.
Here is series that might help. This assumes we have a container with an air-tight lid.
Fill air-tight container with water.
Invert the air-tight container.
Loosen the lid allowing some of the water to escape and creating a partial vacuum.
Re-tighten the lid before all the water has escaped.
Return the container to its upright condition without loosening the lid and apply heat.
Note: This will only be partially successful since once the water is heated the vapor will quickly restore the internal air pressure to and above normal.
Depending upon what is meant by "in normal conditions"
you might be able to take your pot of water to the top of a mountain to boil it. Under "normal conditions" the air pressure at the top of a mountain will be lower than at the bottom of the mountain.