See this FAQ:
https://xoops.org/modules/smartfaq/faq.php?faqid=150

Restore your SQL database
In the new site you will need to have access to your mySQL via the "PHPMyAdmin" application again.
Upload the database by opening the "Run SQL query" window. Select the "database backup" zip file and run the query. The database will be created on the new site.
Now you have to create the same MySQL database user as in the old site and give the correct access rights to the user.

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I did upload the file structure and the database to the new site. I edited mainfile.php. But the new site does not work. I don't know how to "create the same MySQL database user as in the old site and give the correct access rights to the user."

If you backuped your old site and restored that into the new database then the only thing you have to do is change the mainfile.php to reflect the new path and new database name/user.

What exactly doesn't work, could you describe this?