GA, How to modify the requied return to requied risk in a Genetic Algorithm

As you may see, this code have a requied return as a constraint, but I'm triying to modify it, to give a requied risk instead of a requied return.

If someone can help me I would be amazing, I've been trying to modify the code but I haven't had many results.

Thanks.

format long g
filename = 'https://www.mathworks.com/matlabcentral/answers/uploaded_files/835520/Returns.xlsx';

data = readmatrix(filename);

nAssets = size(data, 2);

%Returns and covariance
mu = mean(data);
mu.'

sigma = cov(data);

%formulate the problem/optimization
r_target = 0.004; %r_target is the required return
f = zeros(nAssets, 1);    %there is no constant
A = [-mu; -eye(nAssets)] %besides the returns we forbid short selling
b = [-r_target; zeros(nAssets, 1)] % required return and weights greater/eqauls 0
Aeq = ones(1, nAssets)   %All weights should sum up...
beq = 1                  %... to one (1)

%solve the optimization
fcn = @(w)MaxReturn(w,mu);

[w, fval, flag, output] = ga(fcn, nAssets, A, b, Aeq, beq)

if isempty(w)
   warning('could not find any solution')
else
    %print the solution
    fprintf(2, 'Risk: %.3f%%\n', sqrt(w*sigma*w')*100);
    fprintf(2, 'Ret: %.3f%%\n', w*mu'*100);
end

function f = MaxReturn(w,mu)
  f = w * mu';
end