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PrestaShop's own backup files are saved on your server, and if it turns out your update has gone badly, you can find them in the "Rollback" section of the configuration page for the 1-Click Upgrade module.
Backup files are created as soon as the upgrade process is started, and are available immediately in the "Choose your backup" drop-down list.
Choose the most recent one. The date and hour of the creation of the backup file is are right within the name: Vversion-date-hour-random, for instance "V1.4.9.0_20120907-114024-f85f41a" for a backup of a previous PrestaShop 1.4.9 installation, made September 7th 2012, at 11:40:24 AM.
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