Strip off the old material, check that the deck is in good nick and re-lay weatherproof layer using one of the many flat roofing systems available.
If you go for a standard 3 layer felt system then you with need a tar boiler. This is not a diy job.
If the room below is habitable then you are likely to fall under building regulations remit by way of the 25% rule and would therefore need to upgrade the insulation to current standards i.e. 125mm warm deck etc.
Isn't this like one of those one sentance exam questions that generates 6 sides of A4?
"How to felt a roof. Discuss"
There is lots of info on google about the principles ...... and then ask the questions specific to your roof. Try www.scieroofing.com/flatroofdesign.htm for a start
The problem with such a specific question is that there are too many factors for consideration. Otherwise the answer is as simple as ..... rip up the old felt, roll out the torch on and heat it up.
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