Note:  The "SideBar" page cannot be renamed, however, all other pages of your workspace can be renamed.  Additionally, only users with "Editor" or "Moderator" privileges on a workspace can rename files or pages.

When viewing a page, one of the actions available to Editors and Administrators is the ability to rename it.  If you don't see this option available to you, then you do not have "Editor" or "Administrator" permission on the workspace.