You may separate challa after baking (on weekdays only) if you forgot to separate challa before baking.
Situation You forgot to separate challa from dough made of at least 2.5 lbs. (1 kg) of flour, it is now Shabbat or a Jewish festival.  You want to eat the bread.
WHAT TO DO

• If you are outside Eretz Yisrael:  You may leave part of the challa until after Shabbat or Jewish festivals.  After havdala, separate the challa from the part that you had set aside.
• If you are in Eretz Yisrael:  You may not use bread from which challa was not separated. Once Shabbat or the Jewish festival ends, you may separate challa and then eat the bread.