A method for changing software in a memory of an electronic control unit, wherein each memory address from the overlay memory can be assigned to a memory address in the read-only memory by an assignment information item. During a run time of the control unit, at least a functional part of a bypass routine that is to at least partially replace an original program routine is stored in an address range in the overlay memory, or a jump instruction is stored in the overlay memory as the first part of a bypass routine that refers to a second part of the bypass routine that is stored in an address range accessible to the processor. To activate an overlay functionality the address and/or the address range of the overlay memory are assigned to an address or address range of the program routine to be replaced.