I don't understand if you only wanted a 6 month tenancy why you signed an agreement for 9 months?
You can't just 'exchange' tenancies with someone else. Your tenancy is a legal agreement between you and the landlord and you can't change that. If you did find someone who was willing to take over your tenancy, either now or at the end of the 6 month term, then it would be a new agreement between the landlord and the new tenant. The agent would also charge for making up new agreements, credit checks etc etc.
It would be a waste of money taking legal advice from what you've said here.