Given, EoFe3+/Fe = -0.036V, EoFe2+/Fe = -0.439V. The value of standard electrode potential for the change, Fe3+(aq) + e- → Fe2+(aq) will be:

Fe3+ + 3e- → Fe, E1o = -0.036V
Fe2+ + 2e- → Fe, E2o = -0.439V
Fe3+(aq) + e- → Fe2+(aq); Eno = ?
We can use the following equation to calculate the standard electrode potential for the change:
ΔG° = -nFE°
where:
ΔG° is the standard Gibbs free energy change
n is the number of electrons transferred
F is the Faraday constant (96485 C/mol)
E° is the standard electrode potential
For the first reaction:
ΔG1° = -3FE1° = -3F(-0.036V) = 0.108F
For the second reaction:
ΔG2° = -2FE2° = -2F(-0.439V) = 0.878F
For the third reaction:
ΔG3° = -FE3°
The third reaction can be obtained by subtracting the second reaction from the first reaction:
Fe3+ + 3e- → Fe
Fe → Fe2+ + 2e-

Fe3+ + e- → Fe2+
Therefore:
ΔG3° = ΔG1° - ΔG2° = 0.108F - 0.878F = -0.77F
Since ΔG3° = -FE3°:
-0.77F = -FE3°
E3° = 0.77V
Therefore, the value of standard electrode potential for the change Fe3+(aq) + e- → Fe2+(aq) is 0.770V