If the server is running an old version there’s nothing YOU can do about it, the server Admins need to update their version. However, the PB client on your PC should automatically downgrade to the version running on the server, thereby enabling you to play. Certainly if you could play on this server BEFORE you upgraded your version of PB you should still be able to play on it even if the server hasn’t upgraded it’s version.
If, however, the warning message doesn’t go away and you end up being kicked AND you trust the server you’re playing on, bring down the console (~ or `) and enter:- /pb_security 0. This tells PB to accept a upgrade / downgrade from the server.
Failing that, play elsewhere. Most servers will be running the latest version, so you won’t have this trouble on most other servers.