Editing the default page for a new blog post

I have created a Post in the wp-admin area.

When I attempt to view this post via our blog page, where the list of posts are being displayed, the posts page is using an incorrect format.

How can I edit and customise the page which posts are using, so any older or newer posts are using this template as a default?

I am using the current versions of WordPress (4.7), Upfront (1.6.1) and the Fixer theme (1.3).

Any assistance would be much appreciated!