Change values of table in Access by pressing a button on a form

On an Access form I want am trying to create a button where upon pressing it, it will change all of the values in a field within a specified table to false (i.e. uncheck all of the boxes in that field).

My current form name is DataEntry The Button name is Command77

The name of the table is TestTable The name of the field in TestTable is CheckBox (which has a Yes/No) format.

I've tried this code but it is not working. The error is Variable Not Defined.

Private Sub Command77_Click()
TestTable.CheckBox.Value = False
End Sub