Please help. What resister precisely do I use to reduce a 7.2 volt lipo battery to a 3.7 lipo battery

 

You don't.

Depending on your application, the best thing to do is probably use a DC-to-DC converter. If you are talking about low enough power, then using a linear regulator would be an option.

A resistor is only a reasonable option if all of the following apply:

1) The current draw from the battery is always the same.
2) The current draw from the batter is continuous.
3) You don't care too much about significant variations away from 3.7 V.
4) You don't care about wasting nearly half your power in the resistor.
5) You can vent the heat that is dissipated in the resistor.

 

Here's the answer you asked for:

7.2V -/\/\/\----3.7V----/\/\/\----- GND
----------R1-------------------R2--

R1=3.5/3.7 R2

Select R2=10k and R1=9.46k.

This isn't what you want to do, but you weren't specific in your question, so I only answered your question. You have to select your prefered value of R2.

The diagram above shows two resistors, the midpoint where the 3.7V is obtained and ground.

 

Please help. What resister precisely do I use to reduce a 7.2 volt lipo battery to a 3.7 lipo battery

How much current are you dealing with?
7.2 V minus 3.7 Volts is 3.5 Volts. Volts divided by current gives you resistance. If current will vary you may want to use a voltage regulator. For < 100 mA use the T0-92 case version. For up to 1 Amp use the TO-220 version. Same resistor values.
This works but it wastes about as much power as it delivers.