Presumably, your STYLEREF field is in the page header. In that case, you might use code like:
Sub Demo()
Application.ScreenUpdating = False
With ActiveDocument
.ActiveWindow.View.ShowFieldCodes = True
With .StoryRanges(wdPrimaryHeaderStory)
With .Find
.ClearFormatting
.Replacement.ClearFormatting
.Text = "^d STYLEREF 1 \s"
.Replacement.Text = ""
.Forward = True
.Wrap = wdFindStop
.Format = False
.MatchCase = False
.Execute
End With
If .Find.Found = True Then
With .Fields(1).Code
.Text = Replace(.Text, "1", """My Style Name""")
End With
End If
End With
.ActiveWindow.View.ShowFieldCodes = False
End With
Application.ScreenUpdating = True
End Sub