The water temperature control is the large black knob on the front of the control unit pictured. Under normal operation it is turned to the large mark as pictured. It can be moved slightly warmer if desired, but no more than the next mark.
When the house is not occupied the knob should be turned to the right (counter-clockwise) to the "vacation" mark. Nothing else needs be done to put the water heater on vacation mode.
Note: When moving off of vacation mode it will take several hours before the water heater produces fully hot water.
Note: When using hot water in the bathrooms it will take several minutes before the pipes warm up and you get hot water for baths and showers.