You have to insert a break and then change your text. It's a huge pain, I know, but it's the way that cells and tables as ranges work. Also, your cell text string will be odd because it has a formatting character in it, so you'll have to use Mid
to remove that.
Move text out of nested cell into parent cell
Dim myRange as Range
Dim cellText as String
Set myRange = ActiveDocument.Tables(1).Cell(1, 1).Range
cellText = myRange.Text
cellText = Mid$(myRange.Text, 1, Len(myRange.Text) - 2) 'copy text
myRange.Text = vbNullString 'remove text from cell
myRange.InsertBreak wdColumnBreak 'insert break to allow editing of area before range
myRange.InsertBefore cellText 'places text in the area we created with the wdColumnBreak
Use user-input text as label for nested cell
Dim myRange As Range: Set myRange = ActiveDocument.Tables(1).cell(1, 1).Range
Dim cellText As String
cellText = InputBox("Label Text")
myRange.Move wdParagraph, -1
myRange.Paragraphs(1).Range.Text = vbNullString
myRange.InsertBreak wdColumnBreak
myRange.InsertBefore cellText
The concept is the same, but this allows the user to input the new label string.