I guess you are talking about the two site packages you can choose when installing. There is no easy way to switch between those, they have a lot of different classes, so best thing would be to reinstall with the other site package. If you have a lot of content that you want to move, you'll have change the classes of your existing content before exporting it.
BUT: If it is the website interface you want to test out, please use eZ Publish 3.9.2 witch includes a newer version of it! In that case clean the whole ez Publish installation, and re upload it (and remember to re apply the write rights on extension, settings and var folders).
