I had updated my aadhaar when i was 16. Am I supposed to update it again for NEET UG 2020 as I have completed 18 yrs?

Not in specific for NEET, but yes because you have are legally an adult now, it is good to update your Aadhar card. The Supreme Court has mentioned that Aadhar Card is not mandatory for Academic and entrance examination purposes. So you needn't submit your Aadhar card for admission purposes for NEET. But now that you are 18, it makes it more meaningful to update your Aadhar card with any details that might need change like your photo.