I am using WordPress based web development. I am a beginner, somewhere on YouTube I learned that, to change domain name, install plugin "Better search and replace", and using it replaced my domain name with the desired domain name. But after that, the domain stopped responding. I replaced the whole WordPress file from the ftp server, but in the installation process, it shows the message, "Already Installed, You appear to have already installed WordPress. To reinstall please clear your old database tables first". I have searched ways to do that, but everywhere it's saying to use plugins, which I can't do due to above mentioned issue.
If the database server is available publicly you should be able to use a MySQL client to interact with it.
If the database is only available locally on your web server, you might be able to upload a DB admin script like adminer via filezilla and use that to interact with the database.
If all else fails you will likely have to go back to your instructor and ask him/her to reset the DB for you.
Another option that doesn't require installing anything: clear the database with PHP. Possibly a little easier. See this answer, just replace host, username, password, and database_name with your own from
wp-config.php. Save the file, upload it to your root folder, and access it in a web browser to run it.