Before any uninstall attempt:
A complete backup of the shop and database must be created.
Also, the removal should not be tried directly on the production system without any tests. Instead, try the uninstallation procedure first on a testing / staging system to minimize the risk of disruption.
Options for uninstalling extensions:
Using a version control system (VCS)
If a version control system (Git, SVN, etc.) has been used when the extension was installed, this can now be used to remove the files and undo the changes in the system. Using a version control system is always strongly recommended.
Using the Magento Connect Manager
If the extension was installed with the Magento Connect Manager, you can try removing it that way. However, the Magento Connect Manager is not always 100% reliable and thorough. Also, it may not clean up the database.
Removal by hand
The last option is removing the extension's files and database changes by hand, which means:
- Use the extension package (TGZ file) as a reference for files and directories.
- Make a full backup of the system (files and database).
- Remove the files and directories of the extension from your system.
- In the DB table "core_resource", remove the entry "[extensionname]_setup".
- Check the SQL install scripts in the TGZ to see which changes were made in the database (tables / columns added, etc.) and revert the changes.
Please note: This is a generic article. It does not include a list of files or database tables, because this depends heavily on the installed extension and its version. Refer to the SQL installation scripts to revert the database changes.