Unselect All CheckBoxes From Excel Workbook with VBA Macro

If you have OLEObject-style (ActiveX) checkboxes, then:

Sub terranian()
    Dim o As Object
    For Each o In ActiveSheet.OLEObjects
        If InStr(1, o.Name, "CheckBox") > 0 Then
            o.Object.Value = False
        End If
    Next
End Sub

EDIT1:

If they are forms checkboxes , then the following will work:

Sub clearcheck()
    Dim sh As Worksheet
    For Each sh In Sheets
        On Error Resume Next
            sh.CheckBoxes.Value = False
        On Error GoTo 0
    Next sh
End Sub