To activate a UAN (Universal Account Number), a user must visit the EPFO member portal website and click "Activate UAN". They will then fill in identification details like UAN, Aadhaar, PAN, name, DOB, mobile number, and email. An OTP will be sent to the mobile number, which the user must enter on the website to verify their identity. If the details match, UAN activation is complete and a password will be sent to the registered mobile number.