How to reduce 9v battery to 7v

i have a small water pump that runs on 7v , i want it to run from a 9v battery

Frankly, unless it gets warm, let it run on 9 volts - This will at least use all of the power for pumping water. A 30% over-voltage is unlikely to hurt a small motor. However, if you really want to get to 7 volts, then if you know the current it is easy. A voltage divider would just increase the inefficiancy. A series resistor will do the job.
Assuming 50 ma current, you need a 2 volt drop. R = E/I, so R = 2/.05 or 40 ohms. at a power rating of greater than E*I, or .05 * 2 or 0.1 watt. If you don't know the current rating, then just try something like a 47 ohm resistor and measure the voltage. If less than 7 volts, use a smaller value resistor. If higher than 7 volts, use a larger value.

9V battery is made of 6 cells. Remove on cell away or put a dummy cell with a short wire to make a battery pack into 7.5V . Very close enough to meet your requirement.
Or Choose same shape rechargeable 9v NiCd battery which in facts it was a 7.5V battery......
1.25V X 6 = 7.5V

Use a voltage divider.