Volkswagen Beetle headliner of cabriolet - how to fix round read window

Recently when car was being valeted, the headliner round the read window 'popped' out of the rim.
I tried pressing it back in the rim, but at the last bit, it 'pops out'.

Any idea of how I can see how to rectify this? Was corrected before at VW garage but they now say they don't know how to do this (they said previous VW mechanic must have known how to reclip it).