In the AdminCP, you can go to Content > Edit Custom Pages and select the index from there.
I,
personally, actually recommend not using the AdminCP for doing custom pages, but that's just me, because I want to actively use PHP and Javascript and not have an arbitrary word limit. WYSIWYG editors really kind of irk me with their utter lack of customization and rather unintelligent rendering engines that can't handle HTML elements intuitively. So, if you're like me and would rather edit the index by hand,
see this mod.