There are two ways to erase a portion of your picture.  

If you wish to undo/delete something you just drew, tap on the Purple Arrow at the bottom left of the screen.

  

If the arrow disappears, then you will have to use the Eraser instead.  

If you want to delete something that was drawn earlier on the picture, tap on the Eraser at the bottom left corner.  When the menu shows, tap on the Eraser again and use it to erase specific parts of your picture.