If you've already imported and uploaded a skin:
AdminCP > Styles and templates > Style manager
There you will see the ones you have imported, and can add a skin at the bottom [Add new style]
However, if you wish to change the default skin: AdminCP > Vbulletin Options > Vbulletin Options > Style and language settings. Here, you also have the option to allow users to change styles if they wish.
Hope this helps mate.