How to change code to user input parameter

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I'm wanting to do some sensitivity analysis on some code (sytstem of ODEs, PDEs) and want the user to input a parameter. How would I do that for the existing code? The code would go something like this:

nom par=5;                                 % set nominal parameter value
par=nom par;                               % assign nominal parameter value
[t nom, s nom]=ode45(ODEFUN,[0,Tend],s0);  % nominal trajectory
s nom ss = s nom(length(t nom));           % steady-state concentration
delta=0.05;                                % set deviation (5 percent)
par=par*(1+delta);                         % perturbed parameter value
[t pert,s pert]=ode45(ODEFUN,[0,Tend],s0); % perturbed trajectory
s pert ss=s pert(length(t pert));          % steady-state concentration
abs sens=(s pert ss-s nom ss)/(delta*nom par);   % absolute sensitivity
rel sens=abs sens*(nom par value/s nom ss);      % relative sensitivity

Except for my code, I am calling a function that I want to have user input, and that code that I use it on is plotting some graphs. I have another code that has a bunch of ODE's...any ideas on how or what I should change?