Both jpeg and jfif are suffixes used for the JPEG format, so in most cases jpeg to jfif conversion is not required. You just have to rename jpg to jfif should you need it and that's it. Alternatively, you can use some graphics editor or converter to convert jpeg to jfif, but chances are it will just rename the suffix.
The key difference here is actually in the internal structure of the JPEG file, i.e. EXIF or JFIF. Converters or graphics editors usually save to JFIF, whilst EXIF data are recorded by the digital camera. Unfortunately, both these standards are mutually incompatible and thus in these cases this conversion may actually be not possible.
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