There is a clip that pops out at the top end. Then you can remove the strut.
It will be obvious once you pop out the clip.
The new one comes with new clips, I think.
DO NOT REMOVE BOTH STRUTS AT THE SAME TIME!
I would suggest having someone hold the hatch open or prop it up when replacing the struts. And it is normal to hear something sliding in them.
And look at
http://www.318ti.org/forum/showthread.php?t=3927 for part numbers.