Hm, you’ll have to replace the repository but then clean it up later. I’m not familiar with that process so I can’t really offer any guidance. @jan or @shoothub can possibly provide more input here including why installing version 2.5.1 isn’t working as expected.
@manga411 can you check your Graylog apt list (/etc/apt/sources.list.d/graylog) to see what it says? If the version there doesn’t match what you’re trying to upgrade to, then you’ll need to change it.