copy from one field to other in recordset

[LeoVer]

Hi there,
I have a main form with a datasheet subform (ds2) in another datasheet
subform (ds1). I would like to create a button in the main form that copies
the value from one field in ds2 to another field in ds2, and that for each
record in the open ds2.
I know you can do this using the "For Each...next" statement in a simple
paragraph of code. I understand the logic but not the semantics and I am not
familiar with recordsets.
I would realy appreciate the help,
LeoVer