Leasehold to freehold conversion

I have a flat in a CGHS in dwarka, delhi in my mother's name. We have not got the possesion of that flat. My mother expired a while ago and we are left with 2 brothers and 2 sisters. My mother did not left a will.

2 of the deponents live out of india and thus we have managed to get their POA. Now what is the procedure for converting the flat to freehold and taking possession?