Jyothi, you can select upto 4 convenient exam centres to fill in the NEET application form. And these exam centres once selected cannot be changed. It is not quite clear whether you can change the exam city preferences during the correction window. The exam centres are allotted on first cum first serve basis. And they cannot be changed once allotted as the exam centres are system allotted without human intervention. And you can select any Centre of your choice for your exam. If Shilong is closer to you and you are allotted Shillong as the exam centre, you can attend the exam without any issues.