Any decent editor will preserve the encoding of the file by default, so simply opening it and re-saving in another editor will not fix the encoding. You’ll have to change an editor setting somewhere to specify conversion to UTF-8. How to do this will vary with the editor.
For example, for Notepad++ see here. For TED Notepad there is an encoding option in the File menu. And so forth.
PS. I would strongly recommend using a modern programming editor like vsCode. (To change the encoding to UTF-8 in vsCode, there is a menu at the bottom of the file window.)