How to Change a Nullable Column to NOT NULL

We have a table that has a nullable column (column C1) with some inserted NULL values:

We want to change the nullable C1 column to NOT NULL.

First we need to update all the records that are NULL to some other value, before changing the column to NOT NULL:

UPDATE dbo.Test1
SET C1 = 'w'
WHERE C1 IS NULL

Now the table column C1 has no NULL values:

Now we can change the column to NOT NULL:

ALTER TABLE dbo.Test1
ALTER COLUMN C1 nvarchar(50) NOT NULL