Is it ok to change the salary requirement during a second interview?

During a phone interview with an interviewer whose interviewing style was more like interrogation, I was caught off guard when I was asked what my salary expectations are, and nervously blurted out a number that is too low for the position. Then I was asked to come in for a face to face interview tomorrow...now I'm wondering how I can bring up the subject of salary expectation and tactfully change the initial amount, or if asked about it again, could I simply state the new amount without having to go into explanation?