Please follow below steps to apply or change the Word template to your existing Word document.

1. Open the Word document you will apply template to, and click File > Options to open the Word Options dialog box.

2. In the Word Options dialog box, please (1) click Add-ins in the left bar, (2) select Templates from the Manage drop down list, and (3) click the Go button. See screenshot:

3. In the new opening Templates and Add-ins dialog box, please check the Automatically update document styles option, and click the Attach button. See screenshot:

4. In the Attach Template dialog box, please (1) open the folder containing the personal template you will apply, (2) select the specified personal template, and (3) click the Open button. See screenshot:

Note: If you have saved the specified personal Word template in the default custom office template folder, you can open this folder quickly with folder path %username%\Documents\Custom Office Templates.

5. Click the OK button when it returns to the Templates and Add-ins dialog box.
And now the specified Word template is applied to the existing Word document immediately. See screenshot:

Note: To remove the personal template from existing Word document, please repeat above steps and apply the default Word Template to the document as below screenshot shown:
FYI, you can open the folder containing the default Word Template with folder path C:\Users\your_user_name\AppData\Roaming\Microsoft\Templates.