Anyone Reading this topic and getting started here is a little advice.
You may have edited the theme files with the WordPress editor, that is fine for stylesheets.
Make a mistake with a function code change with the WordPres admin editor and you could find that you cannot login to the WordPress admin area, you could be added to the over 11000 topics here on that scenario.
If you are getting started or asking a question here there are some free tools you should consider.
The cost of all that development software is $0.00 or £0.00 so there is no real reason or excuse to be editing live files with the WordPress admin editor.
FTP is critical if you get into trouble and is just a file transfer system, read up on FTP and use the free software like FileZilla, once setup it is very easy to upload and download config, theme and plugin files or folder.
If you ISP (service provider) does not support FTP upload to your webspace then change your provider.
Advantages are that if a plugin or theme fails to upload or upgrade you can use FTP to transfer the missing files, note that the upgrade deletes all files for the plugin or theme, so if it fails there will be calls to missing functions.
An aborted upload or upgrade of a plugin can break your website.
There are also a few good plugins that do require FTP to upgrade.
Let say you wanted to restyle a part of the theme or change a function, you can then do the changes and testing local and off-line, and just use FTP to upload only the changed files after testing.
HTH (Hope this helps)
Feel free to add other free tools that may help!