There is no easy way to upgrade (migrate) from CB 16 to 18. As soon as a new release comes out, Chessbase usually offers a discounted price to those owning a previous version of their SW, to incentivate the migration. But since 16 has been replace by 17, not by 18, I suspect that no discounted offers exist anymore. You should contact CB and enquire for that.
As far as the data are concerned:
1-Take a full backup
2-Unistall CB16
3-Install CB18
Your databases, games, etc are seen by CB18.
Note that CB18 introduced a database format. You may want to migrate the format of your CB16 DBs into the new one, once you confirmed CB18 works fine.
Is there product for upgrading chessbase 16 to chessbase 18? Or we have to buy chessbase 18 not going for upgrade.
How to transfer my work from chessbase 16 to chessbase 18?
Please guide