Documentation for JIRA 4.1. Documentation for other versions of JIRA is available too. 
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To change the default order from Ascending to Descending so that the latest comments are shown first, follow these steps:
<JIRA_install>/atlassian-jira/WEB-INF/classes/jira-application.properties.jira.issue.actions.order=asc to jira.issue.actions.order=desc