hamid,
I think youve got two options:
1. Include everything you can in your WHERE clause to limit the update to one row. amirs suggestion should work for you for the sample data your provided:
[highlight=sql]UPDATE Table1 SET Value=6 WHERE Name=n1 AND Value=1.[/highlight]
2. Add a primary key to the table - something that will uniquely identify every row. Unless you dont have access to change the table, Im not sure why you wouldnt want to do this. Having a primary key on every table is standard practice - most database engines will even warn you when creating tables without one.
-ner