To manage an AdSense unit you've already created, simply take the following steps:
1.Sign in to your account at http://www.google.com/adsense.
2.Under the AdSense Setup tab, visit the Manage Ads page.
3.On this page, you'll see a list of active AdSense units -- units that have been created or received impressions in the last 7 days. (You can view a full list by selecting the checkbox next to 'Show inactive AdSense units'.) Select the AdSense unit you want to update.
4.Change the settings for this unit, then use the preview to view the changes. Save your changes.
Once you've saved your changes, they'll be visible in this AdSense unit on every page which contains the code for this AdSense unit. Keep in mind that it typically takes about 10 minutes for all our servers to be informed of the change and reflect it on your pages.