You want to modify the settings associated with a GPO.
Open the GPMC snap-in.
In the left pane, expand the Forest container, expand the Domains container, browse to the domain of the target GPO, and expand the Group Policy Objects container.
Right-click on the target GPO and select Edit. This will bring up the Group Policy Object Editor.
Browse through the Computer Configuration or User Configuration settings and modify them as necessary.
You cannot modify the settings of a GPO with any of the command-line tools or APIs, but you can copy and import settings as described in Recipe 9.3 and Recipe 9.7.
The one function that the GPMC tool and API cannot do is modify GPO settings. This still must be done from within the GPOE. You can, however, launch GPOE from within GPMC as described in the GUI solution. Not having a scriptable way to modify GPO settings has been a big roadblock with managing GPOs, especially across multiple forests. Copying or importing GPOs can help with migrating settings across forests.
Recipe 9.3 for copying a GPO, Recipe 9.5 for viewing the settings of a GPO, and Recipe 9.7 for importing a GPO