Quit all apps!
Boot to Recovery Mode either with cmdR (while booting) or in normal GUI mode from within the shell (Terminal):
sudo nvram "recovery-boot-mode=unused"
sudo reboot
The two sudo commands will reboot your Mac immediately to Recovery Mode without further user interaction (i.e. no cmdR shortcut necessary while rebooting)
In Recovery Mode you are already super user: no sudo required and thus not even included in the base system. The security measures introduced with El Capitan (SIP) and Catalina (read-only system volume) are not effective.
In Recovery Mode open Terminal (menubarmenu bar > Utilities > Terminal) and, mount the volume in read mode:
mount -uw /Volumes/[name_of_main_volume]
Example with the default name Macintosh HD:
mount -uw /Volumes/Macintosh\ HD
Then enter:
/Volumes/[name_of_main_volume]/usr/bin/nano /Volumes/[name_of_main_volume]/System/Library/LaunchDaemons/com.apple.nfsd.plist
Example with the default name Macintosh HD:
/Volumes/Macintosh\ HD/usr/bin/nano /Volumes/Macintosh\ HD/System/Library/LaunchDaemons/com.apple.nfsd.plist
Replace
<array>
<string>/sbin/nfsd</string>
</array>
with (here I assume you are following the instructions here and want to add -N to the plist)
<array>
<string>/sbin/nfsd</string>
<string>-N</string>
</array>
and hit ctrlO to write the changes to disk and ctrlX to exit nano.
Enter
nvram -d recovery-boot-mode #according to user3439894 and other users you don't have to execute this command
reboot
to reboot normally.
