An apparatus and method for adjusting an injection device for administering a substance that includes a device housing with an array of fastening points. The apparatus has at least two housing-like shells, each having at least one holding device and at least one blocking device for attaching to the housing. The holding device of a first shell can be attached to a fastening point, and the holding device of a second shell can be attached to a further fastening point. Upon attaching all of the plurality of housing-like shells to the housing, the blocking devices of the plurality of housing-like shells prevent the disengagement of the plurality of housing-like shells from the housing by blocking detachment of the holding devices from the fastening points. At least one of the housing-like shells comprises an electronic assembly and shell attachment to the housing activates the electronic assembly.