Do you mean: "change a [b]parameter[/b] into a manipulated variable"? This essentially allows for changing the value of a 'model parameter' dynamically (e.g. via a controller or an input). A variable is computed (~ output) thus changing it into a manipulated variable (~ input) may require considerable changes to the model.
What you need to do is:
1) change the OBJ definition to interface variable, use the annotation 'manip=1' and set the causality to 'CIN'
2) replace all the occurrences of 'parameters.<OBJName>' by 'interface.<OBJName>'
Which files you need to work on depends on the 'level' at which the parameter is defined:
a) if at a submodel's level, you may need to only work on one file
b) if it is one of the category-specific parameters, you will need to work on many of the matrix-related mal files: *interface.msl, *parameters.msl, ..
The cpp.msl file are of no relevance in this context.