\begin{align} \hat{\mathcal H}= \sum_{i,j} \hat{\psi}^{\dagger}_i H_{i,j}\hat{\psi}_j \end{align} The $\mathcal H$ is the full second quantized Hamiltonian for a system and $H$ is the single particle Hamiltonian in basis $\left|i\right>= \hat{\psi}^{\dagger}_i \left|0\right> $, where ${i}={1,2,\ldots, N}$. And $H$ commutes with $U$ $$ U H U^{\dagger}= H $$ Under a unitary transformation an annihilation and a creation operator transform as \begin{align} \hat{\mathcal U} \hat{\psi}_i \hat{\mathcal U}^{-1} &= \sum_{j} U_{i,j}^{\dagger} \hat{\psi_j} \\ \hat{\mathcal U} \hat{\psi}_i^{\dagger} \hat{\mathcal U}^{-1} &= \sum_{j}\hat{\psi_j}^{\dagger} U_{j,i} \end{align} Here $\hat{\mathcal U}$ is the unitary operator acting on Fermion Fock space. and $U$ is the unitary operator acting on single particle Hilbert space.
I want to proof the last two equations. I have no idea where to start form. Any help is highly appreciated.
Source of doubt: Topological phases: Classification of topological insulators and superconductors of non-interacting fermions, and beyond (section 3.1.1)