how to create backup for update tables structures in existing DB?
Another tool, apart from those already mentioned is ApexSQL Diff

As Mitch has already mentioned, writing update scripts that will synchronize two databases manually is quite a risky job. You can maybe try to compare by creating scripts for all objects and then use a file compare tool to find the deltas between the two schemas and change it. But for even a bit larger databases I would not recommend this method since it’s quite easy to miss some change

For some object types you can use query like this that will give you objects that are not equal but note that this doesn't cover everything.

select O.name, sm1.definition
from DB1.sys.sql_modules AS sm1
inner join DB2.sys.sql_modules as sm2 on sm1.object_id = sm2.object_id
inner join DB1.sys.all_objects O on sm1.object_id = O.object_id
where sm2.definition &lt;&gt; sm1.definition


Disclaimer: I'm working for ApexSQL as a Technical Support Engineer