You'll need a slide hammer to extract the torsion bar. You need to disconnect the shocks. Measure each side from the centre of the hub to top of the arch before you do anything though!
The torsion bar is splined on one end so once its disconnected you move the beam up to the desired position measuring as above on both sides to ensure they match. Relocate the torsion bar in position and refitting is the reverse of removal as haynes tell you!
I'll be honest and i've never done one but i'm going to attempt it on my 205 when time permits. I've got this from the haynes-manual-of-making-it-look-easy and (you may laugh) an episode of wheeler dealers. Its the theory anway.......