I said yes (or no) and changed my mind
You can take a decision back while the other person hasn't answered.
Open the match from the receipt email we sent when you decided. If the other person hasn't said yes yet, there's a "Change my mind" button: it withdraws your yes, or reopens a no.
Once you've both said yes, the introduction has gone out and can't be recalled — the other person already has your name and contact details, and you have theirs. It's on the two of you from there.