The default scrip action is setup to point to channel general:
@ScripActions = (
{
Name => 'Notify Slack',
Description => 'Post ticket updates to slack channel',
ExecModule => 'NotifySlack',
Argument => '#general',
},
);
You can create a new action or update the existing one ( I don’t recall offhand 100% if you can modify an existing action created by code ) to point to your new slack channel.
If you’re on RT 4.4.4 and up you should be able to go to Admin->Global->Actions in the web UI