Go to the account page of that user, and click on "Change role". You can reach their account page by either clicking their username inside a discussion, or on the discussion list. Or just using the search.
EDIT: i fixed(?) it by turning off multi user role extension
i used to see this option but i no longer do. ( the 'change user role' on the member page)- i have rolelistfix selected and in admin settings , i have 'change users role' is checked off.. any ideas ? thanks.