How to change the login name for the dbo account

I was trying to add a new login to a sql server database when i got this error

it was strange because the uses hasn't been already added, so i open each of  the user properties till i found the responsible

The user login i wrote (red square :) ) was already "taken"... y have to get it changed first

I used the following command

USE [DB_Name]
GO

exec sp_changedbowner 'other_user_name'

Then i was able to add my new login to the database