The best way to modify the post templates is to use the decorateWidget method in the plugin API. You can use that to add content before/after the built-in template. There are some brief instructions here for doing that in a theme.
To fully override the template you would use reopenWidget to override the html() method. Some instructions are here.
Adding things before/after is a far more ‘future proof’ way to work, so try and do that if you can. If you have some specifics on what you want to change about the post layout, I’m sure someone will be able to give some more specific advice.