unfortunately, there is no step by step as everyone’s path is different. I upgraded from 2.5 to 3.0 and was looking for something similar. The upgrading information from the documentation site is the place to start, and then you just have to piece the rest together based on your implementation. This isn’t a great answer, but we can’t tell you how to upgrade your environment because we don’t know your environment. The number of nodes you have along with how they are configured play a big role in the upgrade. Are you just upgrading Graylog or are you also upgrading Elasticsearch. 2.5 and 3.0 both support elasticsearch 5.6.13 and 6.x so if you are struggling upgrading both, just upgrade one and then upgrade the other. IF you are running Elasticsearch version earlier than 5.6.13, you need to upgrade to at least 5.6.13 prior to upgrading graylog to 3.0. I upgraded both Graylog and Elasticsearch and it wasn’t a 100% smooth upgrade, but I was able to work through it. If you have specific issues, perhaps I can help. The process listed in the upgrading guide is what you should follow.
If you get stuck, or have issues, post specifics and the community will try to help.