Update Query to run on SELECT records in table

I have a form with two sets of data in, running from one table. I would like
a button that would give the user the ability to update dataset 2 with the
information in dataset 1 on an individual record basis for each page on the
form

Basing this button on a simple update query, updates all dataset in the
ENTIRE table. Is there someway I can build into the query that it only
updates the dataset fields for the one record that it is looking at?

Perhaps more a form question, but can clarify if this does not make sense.

Thanks