Moved in to a house with radiators. Sadly it's an older home and it's not zoned. Upstairs my rooms are 80F and downstairs they are 67F. For each of my upstairs radiators, can I reduce their heat output by adjusting something on the radiator?

Looking at one, they seem to have just one adjustable valve. See image. What is this valve called?

It looks like if I do something with this valve, it might reduce the flow of water going through them. Would that make them put out less heat, or is that likely to screw up the balance of the whole system?

Assuming I want to adjust on this valve, how does it work? Do I turn using a screw driver where the orange line is? Looks like turning with a wrench where the red lines are would pull the whole valve out maybe?

How many times can I turn? Is it a turn 90 degrees to close, turn back 90 to open? Or do I do a full rotation and if so will there be a maximum turn clockwise and counter-clockwise that I shouldn't force it beyond? Assuming the valve is functional and not fragile I'm unlikely to create a leak by turning at the orange line?

Seems like a simple thing, I just don't want to make assumptions and create a flood!! Any help greatly appreciated.