IF your data won’t overflow 4000 characters AND you’re on SQL Server 2000 or compatibility level of 8 or SQL Server 2000:
UPDATE [CMS_DB_test].[dbo].[cms_HtmlText]
SET Content = CAST(REPLACE(CAST(Content as NVarchar(4000)),'ABC','DEF') AS NText)
WHERE Content LIKE '%ABC%'
For SQL Server 2005+:
UPDATE [CMS_DB_test].[dbo].[cms_HtmlText]
SET Content = CAST(REPLACE(CAST(Content as NVarchar(MAX)),'ABC','DEF') AS NText)
WHERE Content LIKE '%ABC%'