Can not change the old template on documents

That is not how templates work in Word. See: What happens when I attach a new template to my document? or How do I copy content and settings from a template to a document? by Shauna Kelly MVP and What is the relationship between a Microsoft Word document and its template?

When you attach a template, your document has access to resources from the new template but this does not make any changes to layout or text.

1. Try creating a new document based on the template.
2. Then copy the contents of your original document and paste into the new document.

See also, my page: Templates in Microsoft Word (https://www.addbalance.com/usersguide/templates.htm)

By the way, you do NOT want to leave the option to update styles from template checked. What happens when I send my document to someone else? Will Word mess up my formatting? by Shauna Kelly, MVP

Please consider this to apply a NEW Template to existing .docx Files:

- Open the .docx document you want to update.
- Go to File > Options.
- In the left pane, click Add-ins.
- At the bottom, in the Manage drop-down, select Templates and click Go.
- In the Templates and Add-ins dialog:
- Click Attach.
- Browse to your new .dotx file and select it.
- Check Automatically update document styles.
- Click OK.