calculate value for resistor in
R9 and R4 set the DC gain of the 2nd stage. In this case, having R4 = R9 = 100k ohms gives a gain of -1.

R9 and C1 set the cut-off frequency of the low-pass filter formed by the 2nd stage. In this case 100k and 2.2 uF gives about 0.75 Hz.

If you want to adjust R9, you should probably adjust R4 as well to maintain the gain of -1. If you want to adjust the cut-off frequency it's probably easier to adjust C1 instead.