You could just delete {1,}, but that might make the code run quite slowly. Instead, try:
Sub Demo()
Application.ScreenUpdating = False
Dim i As Long, StrTxt As String
With ActiveDocument.Range
  With .Find
    .ClearFormatting
    .Replacement.ClearFormatting
    .Text = "[0-9]{1,}"
    .Replacement.Text = ""
    .Format = True
    .Highlight = True
    .Forward = True
    .Wrap = wdFindStop
    .MatchWildcards = True
    .Execute
  End With
  Do While .Find.Found
    If .HighlightColorIndex = wdYellow Then
      StrTxt = ""
      For i = 1 To Len(.Text)
        StrTxt = StrTxt & "x"
      Next
      .Text = StrTxt
      .HighlightColorIndex = wdBlack
    End If
    .Collapse wdCollapseEnd
    .Find.Execute
  Loop
End With
Application.ScreenUpdating = True
End Sub