We have two UPVC doors; one has a "fancy" glass panel which seems to be fixed in with clear mastic, the other just has double-glazed units in a plastic frame ie, just like a window. The latter type just has its double-glazed units held in place by wedges, hidden behind the beading; if yours is like that it will be a simple matter to fix.
Beads off, and re-block (heel and toe) the glass sealed unit. It may be worth renewing the gasket whilst the glass is out. Simple job for a upvc fitter.