Identify and document the reasons for the upgrade.

Study the Ambari upgrade documentation to understand the process. See the Ambari Upgrade Guide.

Develop a written upgrade specification and procedure for your particular upgrade.

Complete the preupgrade verifications in the Completing the Preupgrade Verifications chapter of this guide.

Ensure that high availability is enabled for all components that require it. This is typically the namenode and secondary namenode, but can also include things such as access clients to applications and the NFS gateway.

Create a spreadsheet of tests that you might want to use to validate the health and stability of the cluster after the upgrade.

Simulate the upgrade on a virtual or development cluster so that you can practice upgrading in a test environment before you upgrade your production environment.