how to downgrade postgreSQL version?

I used resolve 15 beta, and upgrade the postgreSQL server to resolve 15 so resolve 15 can open it.
but other workstation install ver 14, and they can't use it any more.
so , how to downgrade the postgreSQL version after I did this upgrade?

You mix up PostgreSQL version , which is likely 9.5 or 9.6 , with the version of the individual databases in there , which is something that only relates to Resolve. One server can host any combination of DR databases and versions in there.
You highly unlikely upgraded your postgresql server version, so i am assuming that you upgraded one of the (or all or your single) database in there from a resolve 14 version to a resolve 15 version.
This can not be undone. You have to restore a backup from before the upgrade (upgrade process tells you in big signs to do a backup) of your database to get access to the projects in there from your DR14 version workstations.
Alternatively if you dont need access from these workstations to any of the existing projects, but only to new projects, you can have resolve (on one of those stations) just create a new DR14 database in your central postgresql server. Just dont have DR15 touch it.

Better practice would have been to create a new database only for DR15 and use that from the DR15 workstation. But that is of course of little help to you now.