An arrangement and a method for modifying the functionality of a security module employ a start loading program stored in a FLASH program memory for reprogramming the FLASH program memory by copying a portion of the start loading program into a main memory of the security module. Data of at least a part of an application program, an appertaining certificate code and identifier data are offered in the communication interface of the security module. The data of the part of the application program are stored on a free memory location of the FLASH program memory when the identifier data characterize a successor status for the stored status. The authenticity of the loaded part of the application program is checked with the certificate code, and given authenticity of the loaded part of the application program, it is stored as valid.