Cannot alter the user 'dbo'. (Microsoft SQL Server, Error: 15150)

Some times we may get the below error while updating the login from dbo to some other login from user mapping window.

1. I have database which is mapped with dbo login and dbo schema. 
2. I tried to update the database with app_user login, then it displays below error:

Cannot alter the user 'dbo'. (Microsoft SQL Server, Error: 15150)

To fix this issue we need to change the default owner to 'sa' and then tried to update the login. 

Use database_name
sp_changedbowner 'sa'