You could use the calculate event for that. How the code exactly would look like depends on where your command button is located and on which worksheet the intended text is stored.
Either way, I would use the event handler on workbook level for that. In case both button and text are on the same worksheet the code could look like this ...
To be pasted in the ThisWorkbook module:
VBA Code:
Private Sub Workbook_SheetCalculate(ByVal Sh As Object)
If Sh.Name = "Sheet1" Then
Sh.OLEObjects("CommandButton1").Object.Caption = Sh.Range("A1").Value
End If
End Sub
In case your button is located on a userform a valid reference to that userform is needed, so the code that launches such a form has to be written in a way it takes that into account. In such a case I suggest to write a custom userform property for that rather than getting direct access to a userform's control.