Airflow: Re-run DAG from beginning with new schedule
Click on the green circle in the Dag Runs column for the job in question in the web interface. This will bring you to a list of all successful runs.
Tick the check mark on the top left in the header of the list to select allinstances, then in the menu above it choose "With selected" and then "Delete" in the drop down menu. This should clear all existing dag run instances.
If catchup_by_default is not enabled on your Airflow instance, make sure catchup=True is set on the DAG until it has finished catching up.