If the document/template was originated in Word, the TOC should have been created using STYLES; specifically the styles of Heading 1, Heading 2 and Heading 3. So, the heading that you have added should be one of those 3 styles (depending on where you made the addition). Here are some links that will help you:

Why use Microsoft Word’s built-in heading styles? | ShaunaKelly.com
Numbered Headings
Understanding styles | ShaunaKelly.com