OK, one of the general rules for modifying Wordpress themes is that you shouldn't modify the theme files directly. If/when the theme gets updated, your modifications will be lost. Instead, for themes other than Montezuma, you should create a child theme.

However, the way that Montezuma is designed, you don't need to create a child theme. Instead, you make changes through the virtual templates. Go to Appearance > Montezuma Options > Main Templates and Sub Templates and read the documentation there. You'll see that the virtual templates follows the Wordpress Template Hierarchy. That is, if you want to make a change to the blog page, you would edit the virtual index.php Main Template (this is where you should have made your change). If you want to change how a single post appears, you would edit the single.php Main Template. The appearance of non-post pages is determined by page.php. Sub templates allow you to make changes to the parts of the larger main templates.

Browse around all of the different sections under Montezuma Options, you will find a lot of documentation there.