Hi
Good news!
Open the theme stylesheet. At the very top, change the name of the theme from WordPress Classic to something related to your site. Feel free to change any of the other info there also. Then save the file.
Using FTP or the file manager in your web hosting control panel, change the name of the folder that the theme is in from classic to something else - your site name is a good choice.
Since WP can no longer find your theme where its looking for it (in the classic folder) you need to reset the theme. Go to the WP admin area, under Appearances. You will see the screen shot of the classic theme there twice. One of them will have your new theme name next to it - make that the active theme. Then go to your homepage, refresh, and make sure this change worked.
If you have any URL's anywhere in posts or in your theme that include the old folder url of /wp-content/themes/classic/ those links will now be broken and have to be updated. Other than that possiblity, everything else should work without any issues.
You can make a screenshot of your theme and replace the screenshot file in your new custom theme folder. That way your theme image will appear on the Appearances page when you go there. The classic theme screenshot is 300 wide x 225 pixels.