If you have a booking at a time you are no longer available, or your customer can't make it, you can reschedule the job by going on your 'Booked' CHP jobs section or to your Calendar, and tapping Reschedule on the job card. You can then select a day and a slot and confirm your booking at the bottom.
In the example shown below, the app knows you're nearby on the 12-4 slot as you have other jobs in the same postcode at those times and will show you a green line under the slot (that's what we call an 'ECO' slot). Once the slot has been selected the slot will turn blue to indicate that. Make sure you have called the customer before rescheduling and that your customer is aware of the changes. If you are happy with your selection, click on 'Confirm booking'