If you’re comfortable with scripting, you could write a short bit of code in a scripting action that will take the existing date in {Start}, add one week to it, then put that new date back into the same field.
If you prefer a no-code option, Make a formula field that adds one week to {Start} using DATETIME_ADD(). The automation’s action would then be an “Update record” action, and would update {Start} with the value from the formula field.