You will have to do it in two steps:
Update the table so that there are no nulls in the column.
UPDATE MyTable SET MyNullableColumn = 0 WHERE MyNullableColumn IS NULLAlter the table to change the property of the column
Update the table so that there are no nulls in the column.ALTER TABLE MyTable ALTER COLUMN MyNullableColumn MyNullableColumnDatatype NOT NULL
UPDATE MyTable SET MyNullableColumn = 0
WHERE MyNullableColumn IS NULL
- Alter the table to change the property of the column
ALTER TABLE MyTable
ALTER COLUMN MyNullableColumn MyNullableColumnDatatype NOT NULL