Make sure you know when option started and ends. The deadline falls on the last day at 5pm and there is no way of getting around it, time is of the essence. TREC or HAR have the appropriate amendment form or just give them a call if you're having trouble. Remeber to look for the Buyers notice to the seller - Termination, and Release of earnest money.
The option period is dictated by the contract or an amendment to the contract. Unless both parties have agreed to changing the date or ending it early, this cannot happen. There must be an executed amendment to do that. Day one of the option period begins the day after both parties have signed the contract and last until the number of days stated on the contract. "If a buyer wishes to terminate the contract during the Option Period, he/she must notify the seller by 5 p.m. local time (where the property is located) on the day that the Option Period ends."