If you have applied manual formatting to text, you can use Find and Replace to search for the formatting and replace it with the appropriate heading style. Generally, though, applying styles is a manual task.

Like any manual task, applying styles may be tedious, but the advantage will be apparent as soon as you want to create a TOC and/or quickly modify formatting of several paragraphs at once.

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional