Our databases are not backward compatible for older versions and once you upgrade your database to accommodate the new version, you cannot just simply install an older version over it.

If you have a backup of your database from eM Client 7 you can skip any export.
If not, you will need to export local data before the downgrade to keep them.

For accounts set up with IMAP or EWS protocol this is not an issue, as setting these accounts anew will synchronize messages and folders back from the server.
For POP accounts and any data in Local folders, export them first via Menu>File>Export and once you downgrade, you can import them back via Menu>File>Import.

The steps to downgrade then are to:

•  Export any local data you need to keep (Menu>File>Export)
•  Uninstall eM Client 8
•  Either delete the database or rename it
•  Install eM Client 7 (You can find older versions in our Release history - https://www.emclient.com/release-history )
•  Restore from the backup (Menu>File>Restore) or create your accounts anew and import your local data back in (Menu>Tools>Accounts, Menu>File>Import)

If you want to just rename your database, the default location is in C:\Users\%user%\AppData\Roaming\eM Client