When you need to convert an image file to another format, the easiest solution is often to use your operating system's default image editing software. In Windows, use "Paint" and on Mac use "Preview".  Note that JPG and JPEG are different names for the same file extension. Look for either in your software's dropdown. Many other programs will also work - open the image in the software you're most comfortable with or if unsure try double-clicking to see the default option on your computer. This should bring up a drop down menu of options for the image file. Your format change will happen simultaneously with you saving a new version of the file. This is helpful because it means you will retain the original file in case anything goes wrong and you'd like to backtrack. You'll need to select an option like "Save As" or "Export" (for Mac) to continue the process. In some software versions, you may need to first "Duplicate" the file (i.e. make a copy) then "Save" that duplicate as a new format. A window should pop up that allows you to adjust the file name as well as the file extension/format. Within the menu dropdown labeled either "Format" or "Save as Type" there should be around 12 options, including ".jpeg" (also known as ".jpg").  Change the name of the file or the location of the file if desired, for instance putting it onto your desktop for easy access. If the extension you want to convert to isn't in the dropdown, try a different image editing software (e.g. Photoshop) or else move on to trying a different method. Once you are done determining the file's name, extension, and location on your computer, click "Save". This will effectively convert your file to the new extension while retaining a copy of the original. Software like "Preview" (and others) can also handle bulk file conversions - simply highlight the files you want converted and right click to see your options.

How would you rephrase that briefly using Vietnamese?
Mở tập tin ảnh. Chọn "File" (Tập tin) ở ngay đầu trình đơn. Save (Lưu) hoặc Export (Xuất) tập tin ảnh. Đổi tên và định dạng lại tập tin. Lưu tập tin.