To format the borders of cells:
Select the cell(s) that you want to format.
Go to the Format menu, and click Cells.
A Format Cells window will appear.
Click the Border tab.
Here, you can:
Select a line type for the borders. Under Line Style, click a style.
Select a color for your borders. Under Color, click a color.
Choose to have regular borders, no borders, or borders on certain sides. To select the visible borders, click the buttons (shown below).
When you are finished, click OK.
The border formatting will be applied to the selected cell(s).
Border Formatting in Advanced Mode
To change the cell border formatting in Advanced Mode:
Select the cell(s) that you want to format.
Click the down arrow on the Border icon (shown below).
A selection of border styles will appear. Click the border of your choice.