Set Field To Have Default Value Of Another
For a one-off update to make saledate the same as saledate1:

UPDATE dbo.test1
SET saledate = saledate1;


You could also do this with a computed column. This would ensure saledate always matches saledate1 (even when the data changes):

-- Drop the existing column
ALTER TABLE dbo.test1
DROP COLUMN saledate;

-- Add it back as a computed column
ALTER TABLE dbo.test1
ADD saledate AS saledate1;


A third option would be to drop saledate, then rename the saledate1 column:

-- Drop the existing column
ALTER TABLE dbo.test1
DROP COLUMN saledate;

-- Rename saledate1 to saledate
EXECUTE sys.sp_rename 
    @objname = N'dbo.test1.saledate1',
    @newname = N'saledate',
    @objtype = 'COLUMN';


This last approach is more complicated there are dependencies on the column. You would need to remove or update those before performing the rename.