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According to this article Use SharePoint's new site templates today! | Microsoft Community Hub, when you apply a site template in SharePoint, there's no automatic way to undo it. However, everything a template adds like pages, navigation items, or themes can be manually removed or changed. For example, you can delete unwanted pages or navigation elements and update the theme to suit your preferences.
-Microsoft has introduced a Site Template History panel to help you keep track of changes:
Go to your SharePoint site > Gear icon > Site information > View template history > This shows every template applied to the site and lets you review what changed.
-Applying a template to an existing site doesn't merge your current content with the new design. Instead, SharePoint creates a new default home page and adds other template-specific content. Your original pages, document libraries, and lists remain intact and can still be accessed via Site contents.
-If you'd like to switch back to your previous home page, you can follow these steps:
Go to your SharePoint site > Gear icon > Site contents > Scroll down to Site pages > find your previous Homepage such as Home.aspx > … > choose Make homepage.
I hope this helps.
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