phpMyAdmin - how to change the value in a field for a group of records?

[#1]

It's a phpBB database and I want to change the value of the poster_id field, after a search to find the ones I need to change.
The Search shows me this SQL...
SELECT *
FROM `phpbb_posts`
WHERE `post_username` = CONVERT( _utf8 'Freddie'
USING latin1 )
COLLATE latin1_swedish_ci
LIMIT 0 , 30
That brings up the first 30 of Freddie's records.
I want to change the poster_id field from -1 to 18, in all of his records (about 15000 of them). How do I do it?

[#2]

I've sorted this out now.
I couldn't see how to do it in phpMyAdmin, so I did it from the command line in the end.