A method of upgrading a pressure generating system comprising: providing a pressure generating system including a pressure generator adapted to generate a flow of breathing gas, a controller that controls operation of the pressure generator, a memory storing a first operating routine, wherein the controller controls operation of the pressure generator according to the first operating routine and a user defined setting, wherein a first set of operating features of the pressure generating system is determined based on the first operating routine, and wherein an internal access key is associated with each set of operating features of the pressure generating system; providing an external device adapted to communicate with the controller; establishing a communication link between the external device and the controller; inputting an external access key to the external device; comparing the internal access key provided by the pressure generating system with the external access key; enabling upgrading of the pressure generating system by enabling the first operating routine to be modified responsive to the internal access key matching the external access key; upgrading the pressure generating system by replacing the first operating routine with a second operating routine and causing the controller to execute the second operating routine so that the pressure generating system operates according to a second set of operating features and the user defined setting; maintaining a database for a plurality of pressure generating systems external to the pressure generating systems, wherein the database includes a serial number unique to each pressure generating system in the plurality of pressure systems, one or more operating routines available to each pressure generating system in the plurality of pressure generating systems, and external access keys associated with each of the one or more operating routines; and updating the database by assigning a new serial number for an upgraded pressure generating system. providing a pressure generating system including a pressure generator adapted to generate a flow of breathing gas, a controller that controls operation of the pressure generator, a memory storing a first operating routine, wherein the controller controls operation of the pressure generator according to the first operating routine and a user defined setting, wherein a first set of operating features of the pressure generating system is determined based on the first operating routine, and wherein an internal access key is associated with each set of operating features of the pressure generating system; providing an external device adapted to communicate with the controller; establishing a communication link between the external device and the controller; inputting an external access key to the external device; comparing the internal access key provided by the pressure generating system with the external access key; enabling upgrading of the pressure generating system by enabling the first operating routine to be modified responsive to the internal access key matching the external access key; upgrading the pressure generating system by replacing the first operating routine with a second operating routine and causing the controller to execute the second operating routine so that the pressure generating system operates according to a second set of operating features and the user defined setting; maintaining a database for a plurality of pressure generating systems external to the pressure generating systems, wherein the database includes a serial number unique to each pressure generating system in the plurality of pressure systems, one or more operating routines available to each pressure generating system in the plurality of pressure generating systems, and external access keys associated with each of the one or more operating routines; and updating the database by assigning a new serial number for an upgraded pressure generating system.