>Hi,
>
>I need to change a name of the default constraint in a table. When I go to this table via SSMS, select the table, open Constraint, right-mouse click on the one I want to change, select Rename. Change the name, press Enter. But the old name reappears. What can I do to change it? TIA.

Try dropping the constraint and re-create.

Found also this http://connect.microsoft.com/SQL/feedback/ViewFeedback.aspx?FeedbackID=462046
not sure if related to your problem

See also http://www.sql-questions.com/microsoft/SQL-Server/32159734/default-constraint-names.aspx for sp_rename syntax.