Without your data nor any code is difficult to specifically help you. Still, a general thing you may do, if you already have the model, is to simply optimize the parameters. An way to do it is:
Par0 = InitialParameters;
[Par,fval] = fminsearch(@(Par)Error(Par,Input,Output),Par0)
function e = Error(Par,Input,Output)
ModelOuput = YourModel(Par,Input);
e = rms(ModelOutput-Output);
Just substitute the variables/functions accordingly.