You can toggle between pen and eraser modes by tapping the icon on the top right corner.
Tapping and holding on this icon will bring up the Menu.

There are various options you can change using the Menu->Options selection,
these are:-

Pen

Pen Width - Sets the number of pixels drawn horizontally on pen strokes
Pen Height - Sets the number of pixels drawn vertically on pen strokes
Pen Ends - Sets the number of pixels draw at the start and end of a stroke

Using the above options the user can achieve a pen style that is similar to a pencil, nib or ballpoint.

Pen Colour - Sets the pen colour
Eraser Size - Sets the size in pixels of the eraser.

Pad

Pad Colour - Sets the background or pad colour
Pad Size - Sets the number of pages that make up the pad.
