Similarly to creating a schedule, updating or cancelling a schedule will work the same whether it's available ✔️ or unavailable ❌. For both of these actions, simply navigate to Setup > Manage resources > working hours.

To adjust an existing schedule, click on the schedule you're looking to amend. The below window will pop up which will allow you to update the schedule as needed. Make sure you hit save once updated.

If you're looking to cancel an existing schedule, hover over the appropriate schedule and click in the ❎ in the corner. If this is a recurring schedule, you'll have the option to delete only the one occurrence or all of them. Select as appropriate and hit confirm.