A .prel is a project file. It is not a video file.
How you output a video from your project depends on which version of the program you're using. Also, it depends on what the project settings are for your project and what resolution you want to output.
In Premiere Elements 13, you use Publish & Share. Go to the Computer page and select AVCHD with the YouTube HD preset.
If you'd like some help with these basics, you may want to check out my free 8 part Basic Training for Premiere Elements tutorials series at:
What version of Premiere Elements and on what computer operating system is it running?
If you post the properties of the file(s) that you are editing in Premiere Elements and want to export with a file with a .mp4 file extension, I will post in your thread suggested customized settings for task that you describe.
You can export a file with a .mp4 file extension using two routes in Premiere Elements (example, using versions 11, 12/12.1, and 13/13.1 example)
a. Publish+Share/Computer/AVCHD with Presets = one of the MP4 presets...there are many choices, you do not have to use just YouTube Widescreen HD
b. Publish+Share/Mobile Phones and Players
Any questions or need clarification on any of the above, please do not hesitate to ask.
Kind of exactly what I said, ATR.