MUMS (Model for Underwater Countermeasure Simulation) is a hydroacoustic simulation model developed at FOA/FOI since the 1990s. The model has been used to assess the outcome of duels with regard to performance, environmental conditions, and tactical utilization. The long term ability to use, maintain and further develop MUMS depends on access to personnel who are well acquainted with the structure and function of the simulation model as well as its input and operation. This report aims to document an effort during the period 2023 to 2025 to modernise the model in different aspects. By making it possible to develop new model parts in other programming languages than Ada, access to tooling and personell has improved. This has also improved the ability to extend the model and its user interface with new functionality. With the suite of tests that has been created it is now possible to systematically test the models function. This has also lowered the threshold for conducting nessecary improvements. The overall ambition with the modernisation effort has to a large extent been reached. This does not mean that the modenisation effort is completed in all aspects. Existing part models are still witten in Ada. The test suite need to be extended with more cases comprising commonly used combinations of platforms, sonars, homers and countermeasures.