The swept-wing boundary layer flow features a three dimensional baseflow profile that exhibits inviscid instability leading to the formation of crossflow vortices. While the initial growth of these crossflow vortices is well predicted by linear stability analysis, the onset of secondary instabilities and the breakdown of crossflow vortices are inherently nonlinear phenomena, and their mechanisms remain unclear. In this study, we conduct a direct numerical simulation of a swept flat-plate boundary layer to investigate energy transfer during the nonlinear stages of laminar-to-turbulent transition. Specifically, we focus on the interscale energy transfer between the most unstable spanwise (crossflow direction) wavenumber and higher ones, aiming to understand how energy is distributed across various scales in secondary instabilities and breakdown of crossflow vortices.