If you’d like to change a table’s borders in PowerPoint 2010, use the Borders button in the Table Styles group on the Table Tools Design tab as shown here:
The above example is a screen shot from my wide-screen monitor. A smaller screen or one that is not maximized or one with a lower resolution might look like the example below. The Borders button is still present, but the word Borders does not display.

Hope this helps!
Kathy