Your Theme files should include a default page template (named: page.php). Your Theme may also have one or more custom page templates, for instance, to display content in wider columns. Or you may have created a custom template of your own.
Edit Page Screen: You can assign a custom template for any individual Page by using the Template dropdown in the Edit Page screen (part of the Page Attributes module):
Select a Template from the list (e.g., My Custom Page).
Click the Update button (or Save Draft if not yet published).