I’ll be very thankful if you can help me to have the similar code (with Fumili) for case of multidimentional minimizing function (in my case function depends on x[0],…x[9]).

Thank you in advance,
Best regards,
Marina Golubeva

Marina,
Take a look at your /tutorials/fit directory. root.cern.ch/root/html/tutorials/fit/index.html
There are a lot of such examples (see myfit.C etc…)
To change default fitter i used this line:
(TVirtualFitter::GetFitter())->SetDefaultFitter(“Minuit2”);

The quoted example for minimization using Fumili already is for a multi-dimensional objective function. You are fitting 2 parameters, so the function is bi-dimensional.
If you want an example for fitting using a multi-dimensional observables (not parameters), for example fitting a Tree with 9 variable, the quoted example can be early extended to this. I can do this if you want.

Also, if you are just fitting a multi-dimensional ROOT object (for example a TH2 or TH3 or TGraph2D) using a
TF2 or TF3, you can simply use Fumili as suggested by the above post:

We’ve changed in your code exampleFumili.C the fitting function of TF1 class
MyChi2Function(int npoint, TF1 * modelFunc, const double *x, const double *y, const double * ey ) :