Target space Diffeomorphism transformation of the Worldline Path Integral

Question

I'm reading Siegel's fields page 278 section IIIB3. I'm interested to know how does the transition amplitude $<\Psi_{f}|\Psi_{i}>$ change under diffeomorphisms of the target space. We use the worldline action $S=\int d\tau[-1/2 e^{-1}g_{mn}\dot{X^m}\dot{X^n}]$ for simplicity and we want to know how does $\int dX_{f}dX_{i}\int DX e^{-iS}\Psi^{*}_{f}(X_{f})\Psi_{i}(X_{i})@pathintegral$