Go to Applications > Accessories > Passwords and Encryption Keys.
Under 'Passwords' there should be an entry for Ubuntu One. You can remove that. Then when you go to System > Preferences > Ubuntu One; it should open your internet browser to the Ubuntu One website. Once you log in there and confirm access from your current PC, it should update the account info.
@Dany
Go to Applications > Accessories > Passwords and Encryption Keys.
Under 'Passwords' there should be an entry for Ubuntu One. You can remove that. Then when you go to System > Preferences > Ubuntu One; it should open your internet browser to the Ubuntu One website. Once you log in there and confirm access from your current PC, it should update the account info.