If you uploaded the file to the file manager with the same name as a previous file then the file WILL be overwritten.
However, due to the way in which Microsoft Windows temporarily stores online files the file will appear to you to have gone unchanged. Everyone else can see the updated file, but you will not until you clear your temporary files. To test this you can either clear the browser cache and delete temporary internet files, or try accessing the file on a different computer.
Click here for information on how to empty your temporary internet files