As a temporary measure to prevent it sagging on your head, you can use push pins to hold the fabric up. :)
I replaced the fabric in a 740 once. Had to remove the whole liner board, which involves removing all the brittle trim bits, and it’s quite difficult to get it out the door opening due to the size/shape. I think I removed the front seats and it was still tricky to avoid bending it.
Once liner is removed, all the old fabric plus the sticky foam residue has to be removed completely otherwise the new fabric won’t stick. I applied the new fabric with a brushable contact adhesive. If you get too much on and press too hard on the fabric it leaves finger prints as the adhesive soaks through the foam. There may be a suitable spray adhesive as well. I bought the liner material from an automotive upholstery place - I think it was down on the southeast side somewhere. Specific headliner fabric made of soft felt-like fabric with a thin foam layer.
If you don’t want to do it yourself, check with Masons Prestige in Frankston as they probably know a local place that could do it or maybe they would even do it? A lot of labour involved so it will not be cheap I’m afraid. Based on your other posts it looks like you already have your car serviced at Masons so see what they say and good luck. It is an annoying problem having that fabric dragging on your head! :)