Register and Install Target Version
Log in to Ambari. Browse to Admin -> Stack and Versions. Click the Versions tab. Click Manage Versions and click OK.
Register new version
Install Target Version
Perform the upgrade
Now you choose the upgrade type. Since I can afford downtime on my cluster, I went with Express Upgrade. There are some checks that are run prior to choosing the upgrade type. If all goes well with checks, click on Express Upgrade and the following confirmation window appears.
Click Yes to start the upgrade.
Errors during the upgrade
Some errors might come during the upgrade. It is possible to read the error message in Ambari, fix it and resume the install. One example of an error I fixed this way was a missing file for Ranger:
error message: FAILED: RuntimeException java.io.FileNotFoundException: /etc/hive/126.96.36.199-8/0/xasecure-audit.xml (No such file or directory)
The fix is the following
sudo cp /etc/hive/188.8.131.52-1245/0/xasecure-audit.xml /etc/hive/184.108.40.206-8/0/
And the upgrade can continue from Ambari.
If you disabled the Service Auto Start option in Ambari, enable it again.
Admin -> Service Auto Start -> Enable
The HDP cluster is now upgraded to version 2.6. One easy check (and my motivation to upgrade ASAP) is to click Add Services and you can see service Druid available to install.