IF EXISTS(SELECT * FROM [dbo].[sysobjects] WHERE parent_obj = object_id(N'TableName') AND object_name(id)=N'DF_TableName_ColumnName')
ALTER TABLE [dbo].[TableName] DROP CONSTRAINT [DF_TableName_ColumnName]
GO
ALTER TABLE [dbo].[TableName] ADD CONSTRAINT [DF_TableName_ColumnName] DEFAULT (0) FOR [ColumnName]