If you've downloaded from the microsoft site you will find its very difficult
to modify as they have used pictures not text.
Easy enough to make your own countdown timer
Draw a rectangle and add the text 02:30, adjust font etc to taste
Now copy and paste - take say 5 secs off ie change to 02.25
Keep doing this till 00:00
Drag a selection box around the lot and use the align tool to line them up
Still with them all selected go to custom animation > appear> delay 5 secs>
Set the slide transtion to auto after 0 secs
Obviously you could substitute any time instead of 5 secs
Hope it makes sense
Did that answer the question / help?
Microsoft Certified Office Specialist