When setting up the service you will have received a confirmation email, within this email is a link that can be used to cancel the one-off reminder, if you wish to change the time of the one-off reminder you will need to cancel the current one and make a new one. You can create a new one-off reminder here
Which service do you need to update?
A scheduled one-off automated reminder
An existing repeating automated reminder/check-in service
Please follow the below link to log in and update a repeat automated reminder/check-in service
A live caller service
Call 0330 058 8244 to update a live caller service