Modify Default value in SQL Server

When you add a column with default value, what happens is that there's a default constraint being created:

create table _temp 
( x int default 1)

sp_help _temp result:

constraint_type constraint_name
DEFAULT on column x DF___temp__x__5A3B20F9

So default clause in column definition is just a shortcut for creating a constraint; default value is not an intrinsic property of a column. If you want to modify default for already existing column, you have to drop the constraint first:

alter table _temp drop constraint DF___temp__x__5A3B20F9 

And then create a new default constraint:

alter table _temp add constraint DF_temp_x default 2 for x

You should drop the DEFAULT constraint and add a new one like this

alter table Mytable
drop constraint <constraint name>
go
alter table MyTable
add constraint df_MyTable_MyColumn default -1 for MyColumn
go

Use sp_helpconstraint MyTable to get constraint names

DECLARE @Command nvarchar(max), @ConstraintName nvarchar(max), @TableName nvarchar(max), @ColumnName nvarchar(max)
SET @TableName = 'TableName'
SET @ColumnName = 'ColumnName'
SELECT @ConstraintName = name
    FROM sys.default_constraints
    WHERE parent_object_id = object_id(@TableName)
        AND parent_column_id = columnproperty(object_id(@TableName), @ColumnName, 'ColumnId')

SELECT @Command = 'ALTER TABLE ' + @TableName + ' DROP CONSTRAINT ' + @ConstraintName  
EXECUTE sp_executeSQL @Command

SELECT @Command = 'ALTER TABLE ' + @TableName + ' ADD CONSTRAINT ' + @ConstraintName + ' DEFAULT 1 FOR ' + @ColumnName 
EXECUTE sp_executeSQL @Command