as Denis wrote, you can do it by choosing a different master page or a theme. There are two ways to achive that:
change the master page or theme (in page properties, see the attach-monoxSupport.png) of your existing page.
Please note that if you change the Master page/Theme this way, MasterPageFile/Theme in markup will not be changed, those changes will be saved in the
2. Through Markup
you just need to specify a new master page file or theme