On the "Pages" tab, click on the page (from the tree menu list) you want to edit.
Click the button "Lock & edit" or "Edit page" (which shows when you have the page locked). See note below about locking pages*.
There are two ways you can select which editable region you want to edit, at the top of the page in the green bar is a drop down menu or in the live preview in the bottom pane you can hove over the content and click.
*Why do you have to lock a page to edit it, this is so only one user can edit a page at once to stop users from overwriting each-others changes.