Any corrections that you want to make, make them as soon as possible after the interview. Dont drag your feet, make it simple and straightforward, no excuses, get to the point:
"Re: correction regarding salary expectation
I thank you for taking the time to interview me for the position of [state the position] on [state the date] at [state the location].
I'd like to make a correction regarding my salary expectations: it is not [state the original figure] but state the figure you currently have in mind]. I apologize for having mispoken and of course, the responsibility for having mispoken is fully mine.
I am really impressed with the way the company introduced itself to me during the interview process and I hope to hear from you going forward"