Article: For image files you can also make the change manually by simply changing the name of the file with your keyboard (i.e. erasing the original extension and typing in another). This is typically a good idea if the current file extension is preventing you from accessing the file's contents (indicated by an error message like "invalid file format").  Your computer uses file extensions as shorthand to understand what software to use when opening files. Be cautious about changing these extensions manually and always back up your file before attempting.  This method can reduce image quality. Using the image editing software "Save As" method is generally preferable. Depending on your settings, file extensions (the 3 letters following the dot in your filename) may not be visible within your standard file view. In Windows, you need to adjust the "View" tab under "Folder options". This is located under "Appearance and Personalization settings". On a Mac look for "Advanced Finder Preferences" to make extensions appear. For more detailed information on file extensions and how to toggle their visibility, have a look at Change a File Extension. Right click your image file and select "rename". Delete the old extension and in its place add in the new extension. For instance, if the file's name is "myimage.png" you can rename it "myimage.jpg" and from now on your computer will treat is as a ".jpg".

What is a summary?
Find the file. Make the file extension visible. Rename the file.