In case you have registered with one of the available social networks (VK, Facebook, Twitter), just click "Sign in with (VK, Facebook, Twitter)" the next time you want to sign into the game.
In case you have registered with one of the available social networks (VK, Facebook, Twitter), and you don`t want to further sign in with your social network account, but rather use an username and password instead, just get a new password to log in with it.