At the bottom of the toolbox, click an eraser shape.
Drag the pointer over the area you want to erase.
The selected background color shows what color the eraser will leave behind. You can change the background color by right-clicking another color in the color box.
You can change a specific color (and nothing else) by changing the foreground color to the color you want to erase and the background color to the color you want to replace it with. Then, when you click the eraser, you can right-click to change the color.
You can undo any number of changes by clicking the Edit menu and then clicking Undo for each change.