The frequency can be changed between 16MHz and 20MHz by modifying the boards.txt file.
By default Arduino IDE will set the fuse bits and set the operating frequency to 16MHz. This is done with the two lines:
nona4809.build.f_cpu=16000000L
nona4809.bootloader.OSCCFG=0x01
To run at 20MHz, change them to:
nona4809.build.f_cpu=20000000L
nona4809.bootloader.OSCCFG=0x02
Note that not all library functions correctly support this change. If you keep it at 16MHz there won't be any issues.