It is possible to edit the backbone \phi and \psi angles indirectly using an option in the Model/Fit/Refine's dialog: “Edit Backbone Torsions..”. When clicked and an atom of a peptide is selected, this produces a new dialog that offers “Rotate Peptide” which changes this residues \psi and “Rotate Carbonyl” which changes \phi. Click and drag across the button 1 to rotate the moving atoms in the graphics window. You should know, of course, that making these modifications alter the \phi/\psi angles of more than one residue.