A file extension is the suffix (like ".txt", ".jpg", or ".docx") at the end of a Windows filename, indicating the file's format and what program should open it. Changing it doesn't alter the file's actual content, but instructs Windows to use a different program to interpret it. Crucially, you must first make extensions visible by opening File Explorer, going to the "View" tab, and checking "File name extensions".

For instance, you might change "Notes.docx" to "Notes.txt" to edit a document as plain text in Notepad instead of Microsoft Word. Another common need is correcting an accidentally saved file, like renaming "Image.jpg" to "Image.png" if it was mistakenly saved with the wrong extension. This action is done directly within Windows File Explorer by right-clicking the file, selecting "Rename", and modifying the characters after the last dot.

While useful for forcing a different program association, changing the extension alone does not convert a file's format – changing "song.mp3" to "song.xls" won't create a usable spreadsheet. This can make files unopenable if changed incorrectly. Misuse also poses a minor malware risk if a malicious file is disguised (e.g., hiding ".exe" as ".txt"). Always back up files and ensure you genuinely intend to change the association before proceeding.