Rename a SQL server

Check the server name:

Select @@ServerName

For a server with a default instance:

sp_dropserver 'old_name'

GO

sp_addserver 'new_name', 'local'

GO

For a server with a named instance:

sp_dropserver <'old_name\instancename'>

GO

sp_addserver <'new_name\instancename'>, local

GO

Restart the SQL Server instance