resistors limit current not voltage. you can use my electrical calculator to figure it out though. if you need a constant voltage you will need a voltage regulator.
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You would actually need two resistors to do that, basically a resistance of around 100Ohms for the to and about 130Ohms for the bottom one.
the picture shows this
if there is a lot of current involved in your circuit or if it is running on batteries then you will want higher values. multiples of these values will give you the same voltage (eg. 100:130 => 500:650 x5)
If you want a constant 5V output from a 9V battery, you can use a 7805 voltage regulator. There are multiple Instructables on how to use them, such as this or this. Basically, you connect your ground for both the 5V side and the 9V side to the center pin, and connect +9V to the side pin. The other side will put out +5V.
how many amps is the said 9 volts?