Choose the new color you want by 1) clicking a square on the color chart, 2) entering decimal number codes (1-255) in the R, G, & B boxes, or 3) entering a hexadecimal color code in the "New Color" box. You will see your "new" color in the "sample" box.

Hit your Return key to activate the cursor. Use your arrow keys to place the cursor over the color that you want to replace & hit your Return key again.