Lock/Unlock a form

Step 1: Show the Developer tab on the ribbon

1. Right click on any of the ribbon commands. For example, right click on Home and then select Customize the Ribbon....
2. Go to Main Tabs box under Customize Ribbon section on the left-hand pane.
3. Select Developer on the Main Tabs box and select OK.  

Step 2: Lock or unlock the form

1. On the Developer tab, select Restrict Editing command.