If you prefer not to use third-party flash tools, you can use official software: Xperia X Performance driver | Developer World
Hold the button while connecting it to the PC via USB. The LED should turn green (Flashmode). Download 502SO 340 R13C Remove Lock ftf
" model and the "SoftBank JP" region to find version (referenced as 340). If you prefer not to use third-party flash
: The tool will begin writing the firmware. Do not disconnect until the process is marked as finished. Alternative Official Recovery Download 502SO 340 R13C Remove Lock ftf