In order to teleport the Capital inside your Kingdom, you will have to use the Clan Capital Teleport item which level coincides with your current Capital level.

Click any tile you would like to move your Capital to, then click "Teleport":

Purchase the necessary item and click "Use":

Note that the higher the Capital level, the more the teleport item will cost.

The next moment your Capital will be teleported, while all clan buildings around it will be archived. You can place them on the new clan territory immediately for free from the construction menu:

Note that clan Forts always require resources to be placed, the farther away from the Capital, the more resources will be needed. Your clan members will not be teleported along with the Capital.

Note that if your Capital is being upgraded, then you will only be able to buy the Teleport of its current level. Be cautious: in this case, once you finish upgrading the Capital, this item will no longer be available for use since your Capital level would change!