How are the relationships between the tables set up? I am assuming that one Job could have many Bills. If so you would just need to make sure you are using the (default name here) [Related Billing] in the Actions section. Under the filter records part it should say something like "Related Billing is equal to Trigger: Billing | Record ID" that will make sure it selects the right record to edit. This is assuming your Trigger is set up correctly to be in the Billing table where a record has had that checkbox you mentioned checked, under additional criteria.
Then in the part below that select the field you want to change and what you want it to be changed to.