Had the same bother when I got mine (2.0lt selespeed SW). Unbolt from underneath, about 7 places, remove front inner wing splash guards, or loosen enough to get you hand in with a 10mm ring spanner to remove the nut holding the front of the skirt, the back is held in place with a plastic push fit fastener. The rest of the skirt is held in place with push fit plastic clips along the whole length onto rivet type pins embedded in the metal sill. Two sided tape hold the top lip in place, this is where you can see where it has come away.
Once removed clean both the sill and skirt and remove all old tape with turps or white spirit, a heat gun can also be used with care! then degrease. To refit, I used 2 sided body work fitting tape and 3M Permanent Spray Mount, (it's used in the graphic trade to mount pictures and artwork) comes in a red tin and is basically Evo Stick contact adhesive. Spray along the sill and skirt allow to dry for about 15 Min's, then attach the 2 side tape onto the skirt, next line up the bolt at the front of the skirt and the plastic plug at the rear, gently push into place, once happy firmly press into place and replace all bolts.
I'm not saying this is the best or only way to renovate, just that it worked for me, tape alone is not permanent enough as it comes away, I've used spray mount for many applications, and it sticks like **** to a blanket!
ps, I had to make sure my skirts fitted back secure as the heads of the pins in the sills had rusted away.