How do I make each stone stand at different height?
How did you set up what you have?
Two ideas spring to mind (three actually upon further reflection)
Make them all a separate object if you haven’t already (in Edit mode -> press P -> by lose parts) and then in object mode “randomize transform” (space to sarch) you can play with only Z axis or add a little random rotation and side displacement as well (make sure to have all bricks selected - also I would make sure to group them or move to a separate layer for easy selection later - you probably did already, but just to be sure)
apply displacement modifier (can work for both single or multiple objects if you set it up correctly). It will be easier to tweak later on, but will influence vertices, not the object as a whole. (some bricks may get a little skewed top
if they are separate objects you can also select only one of them and press O to turn on proportionall falloff (a little blue light will turn on on the lower bar) and next to it you select random. Then you can transform one as normal and by using scroll wheel determin what range will be randomly influenced around the object. And just eyeball it all.
I made bunch of blocks then just copy/pasted.