Her mouth slants into a frown because she wants to say he doesn't have anything to apologize for because some instinct has decided that is fact, but if he tries to argue she knows she won't be able to articulate why. She makes a mental note to fight about it later.

"Yeah, that'd be--" What? Nice, normal, totally something she is looking forward to? "Yeah." Once again, Jyn Erso has nailed it, proving time and time again that she totally knows how to accept kindness and good things with ease.