Changing or introducing settings into a GPO is quite simple. As with everything related to Group Policy, you start inside GPMC. Once opened, expand the trees on the left-hand side until you find the GPO that you want to modify. It is generally easiest to find a particular GPO by expanding the Group Policy Objects folder, as they will all be listed there.

Now right-click on the GPO that you want to modify, and choose Edit...:

This launches a familiar-looking console, the Group Policy Management Editor. This should remind you of the console presented when we first launched gpedit.msc in order to see the underlying settings structure that can exist within the local policy settings. We are now viewing the "guts" of the Default Domain Policy, and any changes that we make inside this screen will take immediate effect within the domain. This is the same process you would take to edit any setting within any GPO; simply right-click on the GPO and choose Edit... in order to see Group Policy Management Editor with the GPO's settings snapped into it: