1. You will receive a Premium Voucher like this: 31837xiekcINHFOPdb9
2. Log into your account: https://uptobox.com/?op=login
(If you haven't account, you just need to sign up free account)
3. Make sure you are logged in, then open page: https://uptobox.com/?op=voucher
4. Put your Premium Voucher and click on "Submit" button to become Premium user.