if data in field prevent change

A

andy

Could someone please tell me (or show me where to put what
code) so that if a user navigates into a field where there
is already data (<> null)? or perhaps (<> "")? or both?
that they are prevented from changing the values in that
field only. I know how to prevent them from deleting a
whole record, but don't know how to do this on a field by
field basis.
Thank you in advance
God bless you
andy
 
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