Solution:
Depending on which kind of hosting you have, you should have access to the site’s files by FTP, SFTP, or through the hosting account’s web-based file manager. You can use one of those methods to replace the theme’s file(s) on the server — or you can download the copy of that file that you modified, change it to fix the last change you made, and then upload it again.
Theme files are located in wp-content/themes/your-theme-name/.
If you’re not sure which type of access you have or which username/password to use to get to the files, the hosting company should be able to help you with that.