Hi Hann,
I think you must get in touch with them immediately and Ask them if you are allowed to rethink your decision, a lot of contracts have 7 days cooling off period, which means you cant get out of it after 7 days, but your contract might have that clause if you would like to cancel your tenancy, I think you must go and check your contract and according to their cancellation clause, you might be able to get out of the contract.
however; you might/might not get your deposit back depending upon the clause.