user.rb

Class User < ActiveRecord::Base
 def update_password(params)
    if authenticate(params['current_password'])
      self.password = params['password']
      self.password_confirmation = params['password_confirmation']
      class << self
        validates :password, length: { minimum: 6 }
      end
      !!save
    else
      self.errors[:current_password] << 'is wrong'
      false
    end
  end
end

## In controller user.update_password(params)

user_service.rb

class UserService
 def self.update_password(user, params)
    if user.authenticate(params['current_password'])
      user.password = params['password']
      user.password_confirmation = params['password_confirmation']
      class << user
        validates :password, length: { minimum: 6 }
      end
      !!user.save
    else
      user.errors[:current_password] << 'is wrong'
      false
    end
  end
end

## In controller ::UserService.update_password(user, params)