How do you change slide numbers?

We create a variable called %pageNO% and add it onto the SlideMaster.  Then a trigger to add or minus one when the next or previous buttons are clicked.  Make sure that they are above the move to next or previous page or the slide will advance before the value is changed.

1. In Slide Master

• insert --> text box 
• insert --> Reference
• Add new variable by clicking on the New icon of dialog box (like pageNumber), if you created before, select that one
• Move the text box and put it in a position you like, you can change font's color, ...

2. In each Slide

• Trigger: Adjust variable "select variable above", Assign to page number you want,  at timeline starts of slide

If you prefer to have an automatic page numbering, add triggers like what Ed mentioned, and skip step 2.