There is no great guide for large upgrades like that. Couple of thoughts:
- Read the official upgrade documentation: Upgrading Guide
- With your data from production, test incremental upgrades between versions. I.e. you might be able to upgrade from 3.4.0 to X.0.0, but it also might break something unexpected. If you go one major version at a time, it will be more likely to succeed. However test the upgrade version steps before you do it in production. There have been problems reported here and the mailing list about huge upgrades like this. You want to be sure, with your real data, before running it in prod.