How do I use this valve properly?
It's called a pressure reducing valve (PRV)
That's a PRV with a shut off and set point. The knob on the bottom is the shut off and the bigger knob with the screw is the set point. Now that you've adjusted it with pressure in the system you may have lowered it and you may need to remove some water to determine the set point. From the picture it looks like you're set at about 2 bar which is about 30 PSI in my neck of the woods 30 PSI on heating system is the upper limit and close to it or much beyond it will cause the pressure relief valve to pop.
What the pressure does
I'm not going to do the exact math but one BAR will push water up about 10 meters. So if the highest radiator in your house is around ten meters high from the point of measurement you would want your pressure-reducing valve set to maybe 1.2 BAR to compensate for errors.
What to do
If the only thing you did was turn the bottom one then no harm done. If you did adjust the top one you're going to need to remove some water but before you do that you're going to need to determine what the correct pressure for your system is. Often on that type of pressure reducing valve the screw on the top is what actually makes the adjustment and the black plastic is just a cap covering the housing therefore turning it has no effect.
In conclusion
It seems like you have not done anything to negatively impact the system but the fact still remains that if it is indeed a heating system the pressure may be set too high (depending on local codes and regulations). If it's a PRV for your domestic system (tap water) then 2 BAR is most likely close to the correct number (also depending on local codes and regulations).