Rather obvious but:

If you don't have a cooling jacket, you can force cooling immersing the tank in water (room temperature or cooling bath). If you can't, at least you can force a flow of cool air to increase heat transfer.

Having the stirring system on will promote homogenization inside and heat transfer.

One simple solution might be to autoclave the fermenter with a minimum volume of water/medium. Check beforehand if pH and dissolved oxygen probes need to be immersed during sterilization. Sterilize growth medium separately (before fermenter & allow to cool or filter sterilize, correct volume and concentration for the volume of medium/water already present in fermenter) and fill fermenter after it has been autoclaved.

Connect your Fermentor with a chiller set at 4 degree and recirculate chilled water. This is the best method. However you can keep the fermentor in running tap water for sometime to cool the contents of the same.

thank you all for excellent suggestions.

You add the medium in the sterlized fermenter and then start the stirrer with chilled water being circulated in the jacket. Note it will depend upon heat transfer coefficient across the material of the fermenter body, and between the medium and the jacket material.