This would be a lot easier for you to do if you had an oscilliscope. It has been literally years since I did this kind of stuff, but to lower the frequency you want to increase either the resistance or capacitance in the circuit. Assuming the potentiometer on the left is the one you're talking about, you can probably just stick another resistor in series with that to tune it to a lower frequency range.
The amount of combined resistance times capacitance is inversely proportional to the frequency. So if you're trying to reduce the frequency by 10x you need 10x more combined resistance and capacitance.
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