Freely controlling wavefronts with metasurfaces has been widely studied in linear optical systems. By constructing phase gradient meta-atoms with nonlinear responses, the wavefronts of high-harmonic fields in nonlinear metasurfaces can be arbitrarily steered by following nonlinear generalized Snell's law (NGSL). However, for incident angles above the critical angle, NGSL fails to predict the generated nonlinear waves. In this work, by involving the reciprocal lattice effect of the nonlinear metasurface, we show a modified diffraction law to completely describe the nonlinear diffraction phenomena. This law is numerically demonstrated and confirmed by designed graphene-based nonlinear metasurfaces in the terahertz regime. Moreover, based on the diffraction law, we designed a nonlinear retroreflector and realized tunable control over a nonlinear wavefront in a single nonlinear metasurface. Our work provides a way to manipulate nonlinear waves and provides a better design of functional nonlinear metadevices.