If you have set a new valid banner on your server, it will be available immediately after you have saved it.
However, the banner is not immediately displayed on your client, as the TS3 client still has the old banner in its cache.
Please click on: Extras –> Options –> Security
Then in the middle under “Cleanup” check the box next to Clear cache on exit.
Now restart the client and reconnect to the server.
The new banner should now be displayed.