In Amazon RDS, how can a disabled master user be re-enabled

First you need to tell the SQL Server to allow everything to be able to log into the server.

CREATE LOGON [YourMachine\Users] FROM WINDOWS;
GO

Then go into the database and setup a user based on that login.

Use YourDatabase
GO
CREATE USER [YourMachine\Users] FOR LOGIN [YourMachine\Users]
GO

Then you can add the new user to the dbo role.

EXEC sp_addrolemember @rolename='dbo', @membername='YourMachine\Users'
GO

All together the script will look something like this.

CREATE LOGON [YourMachine\Users] FROM WINDOWS;
GO
Use YourDatabase
GO
CREATE USER [YourMachine\Users] FOR LOGIN [YourMachine\Users]
GO
EXEC sp_addrolemember @rolename='dbo', @membername='YourMachine\Users'
GO

Just change YourMachine to be your computer's name and Yourdatabase to be your database name.

RDS has recently added a stored procedure to accomplish this:

EXEC rdsadmin.dbo.rds_modify_db_name N'<OldName>', N'<NewName>'

Source: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Appendix.SQLServer.CommonDBATasks.RenamingDB.html