Documentation for JIRA 4.1. Documentation for other versions of JIRA is available too.
If, after upgrading JIRA, you get an error containing 'com/atlassian/jira/license/LicenseFactory', it means that your application server is using old cached JSPs from the previous JIRA version. Please delete the directory where the app server keeps these (the work/ directory in Tomcat; the application-deployments/ directory in Orion; etc), and restart.