Highlight your entire table (To highlight the entire table, click inside the first cell. Hold your left mouse button down, and drag your mouse to the bottom right cell)

With the entire cell highlighted, click on Table from the menu bar

From the drop down menu, click on Table Properties

The Table Properties dialogue box appears

Click the Table tab strip, then click the Borders and Shading button at the bottom

The Borders and Shading dialogue box appears

Make sure you have the Border tab selected

Adjust the settings and click OK.