I don't want to reinstall the new system.
How to upgrade the OPNsense 16.1.20-amd64 to current production version 18.7？
You have to reinstall. Export the configuration (and if needed the captive portal voucher db) and then import the configuration on a new install. Then copy the voucher db and almost everything should be as before as soon as you install your plugins again and reboot.
Oh no~~~~~~~~~~~~