How to rolling upgrade??

[Mr.J]

my cluster A, and cluster B. To upgrade to version 2.6
In what order should I upgrade? 
Journalnode 1 >> Journalnode 2 > Journalnode 3 >> Namenode Std 
>> Namenode Act >> Datanode   ??
Do I also need to upgrade the zookeeper? 
 
hadoop-2.4.1 :  JournalNode, Namenode, Datanode
zookeeper-3.4.6 : zookeeper
 
http://hadoop.apache.org/docs/r2.6.0/hadoop-project-dist/hadoop-hdfs/HdfsRollingUpgrade.html