at bottom of the slider there is a small gromet on each edge,take out these gromets and inside you will see a screw,usually slotted but not always,use a screwdriver and turn anti-clockwise on both and it will lower your slider alowing your door to be lifted out,you may have to replace your rollers as this sounds like your problem,after you have replaced them make sure you adjust your door using same method but this time turning clockwise,also some patios have anti lift blocks in head above slider,if your have them take them out or your slider wont come out and replace them when job is done,hope this helps
shaun www.sdwupvcrepair.co.uk, August 2008