@hautenooz It would not matter how you make the modifications, as long as you make them and save the changes. There should be no problem and everything will work as before.
Check in the plugins section, if you have a caching plugin installed go into its settings and clear the current cache, there should be a button for that. Also clear the browser cache, or test with a different browser.
We could take a look at the website if you could provide a link to your website.
Where do i access the theme files to update the code for instagram rss widget?
note: I had to add a trailing slash in the admin behind the Instagram username. Previously the setting only had the bare username.
@Foxmediakit@gmail.com It will require access to the theme files, usually this is done connecting to the server by FTP
– http://www.wpbeginner.com/beginners-guide/how-to-use-ftp-to-upload-files-to-wordpress-for-beginners/
Also the theme can be updated by FTP, the steps are here
– https://forum.tagdiv.com/how-to-update-the-theme-2/