I need to change the default values of the requirements , the drop-down list , but do not know where to change these values , it is in code or in the database .
I mean the time to describe the terms of reference where we needed to know if the use case, informational or anything else .
Where should I change, delete and insert new values?
Searching your question on google led me to the Mantis bug tracker for Testlink where the requirement drop down feature was suggested (in 2010). There it states there is a config or const file that allows you to configure this.
Checking the Testlink installation manual (See: https://github.com/TestLinkOpenSourceTRMS/testlink-code/tree/testlink_1_9/docs) will refer you to a config.inc.php and a custom_config.php.
You will require FTP access to the server hosting your Testlink instance to adjust this file.