A .prel is a Premiere Elements project file. It isn't a media file, so you don't convert it.
But, if you've added media to your project's timeline, you can publish and share it -- and what you plan to do with the file you output.
How you do this depends on which version of the program you're using. In version 13, use Publish & Share/Computer/AVCHD with the YouTube HD preset. This will produce a high-quality MP4 for posting to YouTube, Vimeo or Facebook or for showing on your computer.