To change it back, using FTP/ SFTP , or your web-host’s cPanel or whatever file management application your host provides, access phpMyAdmin on your host, then find your DB, there check the wp_options Table (Note: The table prefix of wp_ may be different if you changed it when installing), and make sure siteurl field and home field are correct. See this Codex for more info and more options https://codex.www.ads-software.com/Changing_The_Site_URL#Changing_the_URL_directly_in_the_database
If you don’t feel comfortable messing with the database, contact your hosting provider to help you with this or try other methods described in the aforementioned codex
If you haven’t already done, always backup your database before doing any actions, just in case something really goes wrong. You can never have enough backups!
See this codex guide for more info: https://codex.www.ads-software.com/Changing_The_Site_URL
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