The document summarizes upgrading an HDP cluster from version 2.1 to 2.5. It involved setting up new hardware with more resources, performing a blue-green deployment by migrating 500TB of data using DistCp over 3 days, and migrating Hive schemas and other configuration changes. The author recommends upgrading in batches of less than 100 jobs and having help to address errors, as performing a large upgrade alone can be overwhelming.