If all your projects use the same version of a component then there will be no need to update the component. The component will only ever have one version, and all project versions will use this component version.
There is sometimes the need to support multiple components within different project versions. A classic example of this is using third party code. Suppose you are using the third party library 'Widgets' and Version 1 of your product uses the initial Widgets 1.0. You start developing Version 2 of your product using Widgets 1.0. Then you see that Widgets 2.0 has been released with some great features for Version 2 of you product, but you don't want to risk breaking Version 1 (which is now live and doesn't need these new features anyway). To do this you create a new version of the component as described in the section called “Versions”. You update the version to contain the new Widgets 2.0 files. Finally you update Version 2 of your product to use Widgets 2.0 rather than 1.0.
To update a component right-click the project version and select 'Administration | Manage Components'.
Select the component you want to update and press the 'Update Component' button. This will launch a dialog where you can select the component you want to update to.
This will submit a changeset in the project version with the file adds, edit and deletes to update the component.