There are two methods to override a session template:
Go to the "Session Template" page: Schedule > "Cog" icon [top right] > Session Templates.
On the Session Template page you will find a list of all the templates that you have configured on your account. To edit a template, click the "Edit" icon on the far right of the row.
After updating the template, click save and all future sessions created with this template will have the new pre-set values.
At the time of using a template to create a session or class you can override or update any of the fields that populate once the template has been selected and using the "Save As Template" function to override the original template configuration.