File Manager includes text-editing tools for supported files. Select a file and choose Edit; HTML documents may also offer the current HTML Editor option.
cPanel displays a character-encoding confirmation when appropriate. Choosing the wrong encoding can corrupt text, so do not save a file if the content suddenly appears as unreadable or incorrect characters.
Current File Manager documentation also notes that files larger than 1 MB cannot be edited through the normal editor. Download larger files and edit them with an appropriate local tool instead.
Before changing a configuration file, keep a known-good copy so you can quickly restore the previous version.