Answered on July 05,2018
If you are doing it online, it is free of charge.
If you are visiting an Aadhar enrollment centre, you need to pay INR 25 service charge.
Answered on January 30,2019
From January 2019, it will cost you INR 50 to update mobile number.
Mobile number update in Aadhaar requires Biometric Authentication, it cannot be done online. You have to visit nearest Aadhaar Kendra (Locate one from: https://uidai.gov.in/) for this. No document required.