Header Labels are typically used to display the act and/or scene number in the header of each page. They have the ability to hold customized text for successive sections of the script.
The first step is to configure the header so that it contains a Label field (Document > Header and Footer, then click the Label field to insert the Label field into the header.)
Once the header has a label field:
Create Header Labels for the appropriate sections in your script.
This label will appear in the header of every page of the these script from that point forward.
When Act I ends and Act II begins, go to the page on which Act II starts.