To use the custom variables defined, you first need to understand the model and its behaviors. When you load the model, you will see some variables defined in the workspace. To change them all at once, I would recommend creating a MATLAB script that assigns the desired values to those variables. The following MathWorks documentation might be helpful for this : https://www.mathworks.com/help/simulink/ug/change-model-workspace-data.html
To understand how these workspace variables are being used in the model, you can search for the variables inside the model using 'Ctrl + F'. This will help you find all the definitions within the model.
I hope this helps!
