From SQL Books Online ALTER COLUMN cannot be used on a column associated with a default, except that changing the length, precision, or scale of a column is allowed if the data type is not changed.
You have to drop the constraint first then add back after the change. The following example will show how to do from T-SQL.
CREATE TABLE [tbl_15Min] (
[T0] [tinyint] NOT NULL CONSTRAINT [DF_tbl_15Min_T0] DEFAULT (0),
) ON [PRIMARY]
GO
ALTER TABLE [tbl_15Min] DROP CONSTRAINT DF_tbl_15Min_T0
GO
ALTER TABLE [tbl_15Min]
ALTER COLUMN [T0] [int]
GO
ALTER TABLE [tbl_15Min] ADD CONSTRAINT
DF_tbl_15Min_T0 DEFAULT 0 FOR T0
GO