There are two ways to edit your custom page title (what shows in the Navigation bar and the page header). The second way includes how to edit your page slug.
The first way (just Title):
Within the custom page editor, click on Edit Info located on the floating remote. There you will be able to edit the Page Title, Description, and Keywords.
The second way (Title and Slug):
Click on Manage Content located on the left side toolbar > click on the gear icon that appears to the right of each custom page > click Edit Page Title and/or Edit Slug.