Adobe Premiere does not come with any way of producing MPEG files.
There is no way to "convert it." YOu will need to recompress it. I always recommend AGAINST recompression because it damages quality. THere is no way to re-encode an mpeg without loosing quality.
But if you have to, the easiest thing would be open it in Tmpeg (
www.tmpgenc.com), set the settings you need, and simple compress a new mpeg.