Create another Layer. Then when the user presses the options button, for example, set your Help Screen layer to visible. Then when they close it, set that layer back to invisible.
Here is an example of what I use to switch between my level select buttons.
<img src="http://img842.imageshack.us/img842/7282/support014.jpg" border="0" />
EDIT: I should also mention that I just controlled a variable with the left/right buttons and made display changes based on that value. You don't need to do this. Just use On Touch > Help Button. Or w/e it is you are using.