Login to your business catalyst admin panel, Go to Site manager-> Page Template-> open the template and switch. Then change it.
If you do not see Page option under Site Manager, In Admin Panel, on your Right Hand side top, Mouse over on your name and Go to "My Details". You will see a check box "Enable online content editing". Check this box, Click save and logout. Login again, and goto site manager, you will see some more options like File manager
Hope this helps