The form stores data in a table. You need to add a date/time field to your
table to record the date and time a record was last updated.

1. Open the table in design view.

2. Add a new field named (say) LastUpdate, type Date/Time.

3. Save and close the table.

4. Open your form in design view.

5. In the Properties sheet, set the Before Update property of the form to:
       [Event Procedure]
Note: Make sure the title bar of the Properties sheet says "Form."
It won't work if you set the property of a text box.

6. Click the Build button (...) beside the property.
Acess opens the code window.
Set up the code like this:
   Private Sub Form_BeforeUpdate(Cancel As Integer)
       Me.[LastUpdate] = Now()
   End Sub