>>>Hi,
>>>
>>>How do you update a database structure (default keys, constraint, etc) installed on the customer site with the changes you have made to your copy of SQL Server database? That is, I understand that I have to create some script and send it to the customer and instruct them to run this script, correct? How do you create such a script? TIA.
>>
>>We're using RedGate SQLCompare.
>
>Is this a commercial product? If so, is there a way to do it without buying a tool for that?

VS2010 Premium has an excellent database and data compare/synchronize functionality.

Before worrying about that, you can right click on most anything (database/table/key/constraint/etc) in the object explorer tree in SQL Server Management Studio and pick a script option and it will write a script for you.

Another free tool that may be of help is SSMSTools - it's available here.. http://www.ssmstoolspack.com/