6. Upgrade ZooKeeper

Execute the following command on all the ZooKeeper nodes:

For RHEL/CentOS/Oracle Linux:

yum upgrade zookeeper

Replace your configuration after upgrading. Replace the ZooKeeper template configuration in /etc/zookeeper/conf.
Start ZooKeeper. On all the ZooKeeper host machines, execute the following command:
```
sudo su -l $ZOOKEEPER_USER -c "source /etc/zookeeper/conf/zookeeper-env.sh; export ZOOCFGDIR=/etc/zookeeper/conf; /usr/lib/zookeeper/bin/zkServer.sh start >> $ZOOKEEPER_LOG_DIR/zoo.out\"
                    
```
where
- $ZOOKEEPER_USER is the ZooKeeper Service user. For example, zookeeper.
- $ZOOKEEPER_LOG_DIR is the directory where ZooKeeper server logs are stored. For example, /var/log/zookeeper.