You don't... .prel are project files, not video files... what you do is export (publish/share) your project to a new video file
The Elements Tutorial Links Page http://forums.adobe.com/thread/1275830 should help
What version of Premiere Elements are you using and on what computer operating system is it running?
In the absence of that information, I can give you only a generalized answer using specifics for Premiere Elements 12/12.1 on Windows 7, 8, or 8.1 64 bit.
1. Publish+Share/Computer/AVI and then decide on what preset you require. You can customize the export settings under the Advance Button/Video Tab and Audio Tab of the preset.
The major hurdle you may encounter is finding AVI in that area, no matter what version of Premiere Elements that you have. Under Publsh+Share/Computer there are many choices.
Adobe Flash Video
But when looking in this area, many get the impression that there are only 3 options Adobe Flash Video, MPEG, and AVCHD because they have not scrolled down the list to see what they need which is probably at the bottom of the list. Note the thin scroll bar to the right of the choices. If necessary, I will post
Please review and consider and then we can decide what next.