You should have a tool that you attach and then wind up or down, there may be a locking nut to losen first, sorry if this is not too clear but best i can describe.
The rear springs should be sitting on an adjusting plate that'll look a little something like this:
Often people take out the adjuster (blue bit in this case) to get an extra few mm, if this has been done you will be unable to adjust height without it.