How do I heat up pre-made crepes with cognac?
Given that you ask about flaming cognac, I'm going to assume that you're making Crepes Suzette.

If that's the case, it's not complicated.  Follow the recipe I just linked; you'll see that even though it includes making crepes from scratch, that's a separate, first step.  You then heat the crepes in the orange butter and sugar.  if they are too dry to fold, heat them briefly in a dry pan over low heat.

Flaming with cognac is the last step, and it's done by setting fire to the cognac in a cup and pouring that over the crepes.  Again, this is all in the classic Pépin recipe I linked above.