Yes, you can. Please email to Contact_YPS@hpb.gov.sg to cancel the appointment.
After taking the immunisation from your own doctor, you may inform your doctor/clinic to update the records via the National Immunisation Registry (NIR) website. Alternatively, you may email a copy of the certificate of vaccination to firstname.lastname@example.org.
Please also keep a copy of the immunisation certificate in the Health Booklet.