A new method is disclosed, for upgrading the software in a real-time distributed software system comprising several processes running in parallell. The method involves a preparatory procedure in which the new software is installed. Then, the software is activated, the components ordered to save configuration data and transaction data in a database, whereupon the components are stopped. When the preparatory procedure is completed, a restart procedure is invoked. This involves selecting start level (full upgrade involving all components, or a partial upgrade concerning just one or a few components). The modules concerned are started from the new software version. On each start level a number of start phases are defined. These are sequential steps performed during the restart operation. Each module subscribes to a number of these phases. Some of the phases are defined as synchronisation points, here called service levels, which all modules have to arrive at before the process is allowed to commence. During this process, each module is given the responsibility of converting any configuration data or transaction data belonging to it, if needed. The conversion is done by reading the old data from the database mentioned above and converting said data to the format required by the new software.