You can cancel a scheduled one time workflow by pausing or deleting it. If you are looking to change the scheduled run time of the workflow, you can choose to edit it and select a new date. First go to Settings > Workflows.

Pause a scheduled workflow

Click the 3-dot menu for the workflow you'd like to pause and select Pause.

Delete a scheduled workflow

Click the 3-dot menu for the workflow you'd like to delete and select Delete.