Nothing now, not in time for Christmas anyway. Article below in the Daily Mail with Monty advising what must be done to achieve new red bracts

http://www.dailymail.co.uk/home/gardening/article-1083884/Ask-Monty-How-I-poinsettias-leaves-turn-red-Christmas.html

I've just put mine in the bin - it survived even with 4 red bracts still left on it, but I couldn't be bothered to go to the performance required to get it to flower and produce red bracts again, specially when they're only a couple of quid in Waitrose/Tesco wherever.