Site definitions are used like blueprints when a new site  collection is
provisioned. Once the site is provisioned there is no way to change  the
site definition  and have existing  sites pick up the changes. One potential
workaround does exist. If the changes  you want to make to the site
definition can be added via a feature then you can write a feature and add
the feature to the existing site definition. Then you can manually activate
the feature in existing sites and get the same effect as re-applying the new
site definition.

I believe you are talking about Feature Stapling,right?

Not necessarily. Feature Stapling (Feature site  Template Association) is a
way of adding a feature to an existing  site definition. This is normally
used because you want to add something to one of the built-in site
definitions without breaking the rules by editing them. In this case the
Yang Li wants to add something to an existing custom site definition. You
could use Feature stapling, or just add a reference to the Feature to the
WebFeatures or SiteFeatures element of the site definition. With a custom
site definition  there is no reason not to edit it further, but the change  to
the site definition won't do anything to an existing site. By packaging the
change as a Feature and adding the Feature to the existing site definition
you have the change in a form where it can be activated on existing sites
also. But in a custom site definition you can add the new feature either by
referencing it directly or through Feature Stapling.