TCanvas *se = new TCanvas;
TH1F *h = new TH1F("h","test",100,-3,3);
h->FillRandom("gaus",3000);
gStyle->SetOptStat();
h->Draw();
se->Update();

TPaveStats *ps = (TPaveStats*)se->GetPrimitive("stats");
ps->SetName("mystats");
TList *listOfLines = ps->GetListOfLines();

TText *tconst = ps->GetLineWith("RMS");
listOfLines->Remove(tconst);

TLatex *myt = new TLatex(0,0,"Test = 10");
myt ->SetTextFont(42);
myt ->SetTextSize(0.04);
myt ->SetTextColor(kRed);
listOfLines->Add(myt);

h->SetStats(0);

se->Modified();
return se;

gROOT->GetListOfCanvases()->Draw()