For the first request, you can use
<pre> UPDATE iweb_conf_demo
SET Attendee_ribbons = NULL</pre>
This will clear ALL information from that field in the table. Also, this step seems unnecessary as you will be replacing the values anyway.
For your second request, based on what I could gather about the structure of the DB, You could use something like this:
SET iweb_conf_demo.Attendee_ribbons= Gen_table.code
INNER JOIN Gen_table.code
ON (iweb_conf_demo.Attendee_ribbons.ID = Gen_table.code.ID)</pre>
This will set the attendee_ribbons value to the corresponding code value where the IDs match in both tables.
If I made an incorrect assumption about the structure of your database, please provide the corrected details and I will gladly alter the suggested scripts to fit your environment.