SELECT email FROM users;

+-----------------------+
| email                 |
+-----------------------+
| abc@gmail.com         |

UPDATE users SET email = 'def@gmail.com' WHERE email = 'abc@gmail.com';

SELECT email FROM users;
def@gmail.com

user = User.new
user.email = 'xyz@gmail.com'
user.password = 'changeme'
user.password_confirmation = 'changeme'
user.save!
User.pluck :email
=> [ "def@gmail.com", "xyz@gmail.com"]
user.email = 'mno@gmail.com'
user.save!
 => true
user.email
=> "xyz@gmail.com"