Update a value in table when value in another table posted

How to set up an action when one table form is saved . so that .it updates value in another table by action

I have two table Car rented and change cars. car rented is parent and change cars(child) has a ref field to car rented
in car rented . the.fields are car rental no (key) and car name and some other fields,
sometimes client may request changing cars , in such cases change rental form is filled with old car name and change car name .when this form is saved it should automatically update the car name in car rented table
how to do that with actions …the action should trigger with saving of form in change car table