The liner is held in place by an edge piece that has a clamping effect on the material. The material is pressed up into the clamping channel. The easiest way to remove it is to use a flat putty knife and slide it into the channel where the material is. Start at a corner and slowly work the material out as you gently separate the clamping channel with a sideways pry. I use a putty knife that is about 3" wide and the edge has been rounded as to not cut the material.
To re-install use the same putty knife and press the material up into the channel. once you get a feeling of where to contact the material with the putty knife as you press it into the channel it will re-install quickly and at the correct tension.
Just go slow.