To edit a Keyword, from the "Manage Campaigns" tab, select the Campaign, and then the Ad Group in which the Keyword lives.  Find the Keyword you want to edit, click the "Actions" button, and select the "Edit Keyword" link.  From here you will be able to edit the Match Type and Cost Per Click (bid) for that Keyword.  From the Actions button, you can also Pause, Delete or Activate the Keyword.