There are 2 ways. If you are aiming for an:
- Acceptance: write an email of interest indicating why you are a good match with that particular program, and specify at least 2-3 laboratories (no more than 5) where you could easily pursue the research of your interest.
- Withdrawal: write an email thanking the program for inviting you to interview, but indicate that you need to withdraw because another program matches better with your research interests.