For most skins, there is a skin.
js file that is used to add customizations like the one you want. If you want the change to be applied to all skins, then you'll want to edit the BaseSkin.
js (under <src>/ZimbraWebClient/WebRoot/skins/_base/base/). In this file, there is a ZmSkin class defined. In this constructor of this class, there is a "help_button" hash defined. Simply add to this hash like so:
Code:
help_button: { style: "link", container: "quota", url:"http://www.zimbra.com" }, You'll need to redeploy the web client to see the changes take effect. To do so, go to <src>/ZimbraWebClient and type "ant prod-deploy" to deploy the production build.