Ok, that's pretty much covered by my generic answer.
Find the backup file and put it somewhere safe. Stop, and then uninstall Confluence 4.3. Drop all tables from the database. Re-install Confluence 4.2. Log into it and look for the "import backup" option in the admin screens.
Have a quick look through https://confluence.atlassian.com/display/DOC/Site+Backup+and+Restore
If you know whare this file is, just install a new trial JIRA istance (same old version) and try to import it. In case you don't have it, perhaps you should raise a support ticket in order to find a better option.
I believe that you'll have to clone the database and fix the upgrade in a second instance.
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