BootsnAll Insiders

I have a 3 monthes ticket purchased from Ndjamene(Chad),it was Ndajamena -Paris-Ndjamena, my return trip was scheduled on the 14 of this month (March)but i would like to pospond it to another date, Pls. advice me how can i do that from here in London,and what should be the equivellent fare if any.