How can I change the template for existing blog posts?

Changing the template in the settings will replace the template for all existing blog posts and can be updated in HubSpot Settings for the blog

The Blog Post template can be changed in HubSpot Settings here:
Settings  → Website → Blog → Templates

If your blog is currently live with another theme or template in HubSpot, completing these steps will immediately change the appearance of your blog.

Make sure your header, footer, and theme settings are configured before changing the blog post template.

To change the template, in HubSpot Settings under Tools, click Website, then Blog, and select the Templates tab.