Note that the PLL is only rated to 384Mhz and we only qualified the boards at +320MHz. If you try to program something the board can’t handle, you’ll probably need to use the “safe” bootloader to recover, so you should learn to use it before going nuts with the clock rate.
The problem with just using those to change the clock is that the rest of the Arduino code needs F_CPU for UART Baud Rate, delay(), etc, so redefining this is the most reliable way to play with this in Arduino-land. See:
(on linux, this is placed in ~/.arduino15/packages/sifive when you download the package).