Hi,

1. You can use “Find and Replace" in word , you can choose the heading style in Format for Find Style and Replace Style.

2. You can also promote or demote the level of all Headings

2.1 First of all, click “View” then choose “Outline”.
2.2 In “Outline” view, you press “Ctrl+ A” to select the entire document.
2.3 Next click the single right arrow in “Outline Tools” demote the level of all headings in the document. Or click the single left arrow to promote heading level. Each click promotes or demotes one level a time. So, make one or multiple clicks accordingly.
2.4 Lastly, click “Close Outline View” to exit.

3. or you can use VBA
Sub PromoteOrDemoteParagraph()
Dim strRule As String

Selection.WholeStory
strRule = InputBox("To promote heading level, enter p" & vbNewLine & "To demote heading level, enter d")

If strRule = "p" Then
Selection.Paragraphs.OutlinePromote
Else
Selection.Paragraphs.OutlineDemote
End If
Selection.Collapse wdCollapseStart
End Sub

Here is teh article I find this way,

https://www.datanumen.com/blogs/3-ways-batch-promote-demote-heading-levels-word-document/

Hope it helps.