Exchange Of Login For Database User

I have a database with 2 users - 'dbo' and 'user1'.

Currently:
=> 'dbo' is tied to login - 'login1'
=> 'user1' is not tied to any login.

I want to change the login tied to the database users to:
=> 'dbo' tied to no login
=> 'user1' tied to 'login1'

I try to use:
sp_change_users_login 'Update_One', 'user1', 'login1'
But it give me an error said 'login1' is already tied to a user.

Then I try to remove the login tied to 'dbo' with this:
sp_change_users_login 'Update_One', 'dbo', ''
Then I get another error said that 'dbo' is a forbidden value for the
login name parameter.

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Previously, all the while 'dbo' is not tied to any login and 'user1' is
tied to 'login1', but yesterday I restore the DB from the backup file
bring back from customer side, then I saw the login changed to the one
I mention above, I'm not sure whether is the DBA from customer side
change it or what, but is there any way for me to change the login back
to the state I want?

Thanks.



Peter CCH