Changing the name of a tree

Is there a command in root to change the name of a tree?

You can change the name of any ROOT object deriving from TNamed (case
of a TTree) via the function SetName
Note that this will change the name of the Tree for the object in memory.
If you want to change the name for an object stored in a file, do something

TFile f("myfile.root","update"); TTree *T = (TTree*)f.Get("oldname"); T->SetName("newname"); T->Write();