If the downtime is not an issue, just shut down the cluster (bin/stop-dfs.sh), unzip the archive with hadoop-2.6, copy configuration files that you've changed in 2.4.1 into 2.6, change the hadoop link so that it's now linked to the new hadoop-2.6 folder and run this command:
bin/start-dfs.sh -upgrade
After this, the cluster should be started and working.
You may find more information about hadoop upgrade here