As Andrew Eisenberg said, go to Window --> Preferences --> General --> Editors --> File Associations and ensure your choices for editor for each file type are correct according to your desires.
However, for Eclipse CDT most of your file types are set in Preferences --> C/C++ --> File Types instead.
Also, like he said, you can use the right-click menu to choose your own editor, including an external system editor, such as Sublime Text 3, Atom, Visual Studio Code, Notepad++, etc.
Right click on the file --> Open With --> Other... --> choose an "Internal editor" or "External program" to your liking, optionally checking the box at the bottom to make this the default choice next time.