An arrangement and a method to retrofit a wind turbine is provided. The wind turbine is retrofitted by at least one of its blades. The blade is prepared for a connection with the hub of the wind turbine. An extension unit is designed and prepared to increase the surface-area of the blade. The extension unit and the blade are designed and arranged to interact with each other by an opening. Thus the opening might be arranged inside the blade and/or inside the extension unit. The extension unit is connected with the blade by an adhesive, which is applied at least partly within the opening.