I don't know a one click option unfortunately.
There are two ways I am aware of:
1) You create a new empty pgn database and copy the games from the cbh file to your new pgn.
2) You open the cbh, select all the games you want in your pgn and right-click the selected files, then choose "Output", "Selection to Textfile" and choose the option "pgn".
If there is a way to do it more conveniently, I have yet to find it.
Sidenote: Years ago, there were some free conversion tools / programs on a private internet site. I never bothered to download them because I don't need it that often. Might be worth to check if these programs still exist, if you use it a lot. Edit: One such converter can be found here:
http://cbh-to-pgn.soft112.com/I have not tried it myself. Can't guarantee it works. Can't guarantee it's virus-free.