Separate Post and Page Files??

The theme I’m using has only one file for posts and pages, the index.php file.

I want separate files for each, one for posts and one for pages so that I can format them differently.

1. How do I create those separate files and get wp to recognize them when I draft something?

2. Any easy way to go back and “re-assign” the new files to old posts and pages (to adopt any updated formatting)?