Hi,
Here's the link to the help page of Adobe Story which describes how to add/remove Acts(or Parts) in Story: http://help.adobe.com/en_US/story/cs/using/WS2562f7e2f5c92d92708a360d1 35ebd85a5f-7ff9.html#WS2562f7e2f5c92d92708a360d135ebd85a5f-7ff8
Hope this helps.
Regards,
Sunny