by Johnny Lo.
You need to upgrade it to 2.2 as a stepping stone before you can then upgrade to 2.5. Some core part of moodle has been completely rewritten for 2+, you cannot restore courses from 1.9 directly to 2.5.
You would also need to check for any plug-in compatibility.
The articles below about upgrading provide very good guidelines.
http://docs.moodle.org/22/en/Upgrading
http://docs.moodle.org/25/en/Upgrading