You change frequency with a combination of pre-scale and TOP. Depending on the mode you set the TOP value is stored in OCRxA or ICRx. Note that the prescale value will also affect your pulse width. If you have to change the prescale to get the full frequency range you will need to change the PWM value to get your desired pulse width.
The base clock is 16 MHz so 16 clocks per microsecond so 32 counts per pulse with a prescale of 1. If you go to a prescale of 8 you are down to 4 counts per pulse.
50 kHz is 320 counts at a prescale of 1. That means you have to use a 16-bit timer or a prescale. If you use a prescale of 8 you can use an 8-bit timer and would set TOP to 40.
450 kHz is 35.555 counts at a prescale of 1. That will work fine with an 8-bit timer. Of course you can't use 35.555 so you should use 36 (444.44 kHz) or 35 (457.143 kHz).