Rowguid Column

I have a question regarding 'rowguid' column.
I have created a table that includes a rowguid column.
The syntex that i used to create the table is as below:

Create table mytable
(
process_id uniqueidentifier,
Date datetime,
processdesc nvarchar(300)
)

Now, the inserts in this table are giving me the values as nulls for process_id column.
Then, I came to know the correct syntex would have been
Create table mytable
(
process_id uniqueidentifier default newid(),
Date datetime,
processdesc nvarchar(300)
)

default newid() is given in order to generate a unique value to the column.
Now i dont want to recreate the table aftre dropping.
I just want to alter the table by just adding 'default newid()' to it.

How can i do that????

thanks

Try

ALTER TABLE .. ATLER COLUMN .. default newid()

If that doesn't work, then update should work

You can run UPDATE with set process_id=newid()