Inspect the water stained area for any permanent damage. If ceiling material is lose or falling off, then the stain area will need additional repairs before being whitened. Make any necessary repairs.
Fill a spray bottle half full of plain tap water. Fill the bottle the remainder of the way with common household bleach.
Cover carpets and furniture underneath the affected area with plastic drop cloths to protect them. Put on an old shirt or a smock, eye protection and rubber gloves. Stand on a step stool, and spray the stain with the bleach and water solution. Make sure the sprayer is on the wide spray setting, not the stream setting.
Allow the stain to dry for half an hour. Repeat step 3. Allow the stained area to dry once again for at least 30 minutes.
Repeat Step 3 a third time if necessary, concentrating the spray on just the areas where any stain is still visible. Allow to dry thoroughly. After three sprayings your stain should be completely gone.