There is not simple downgrade path supported. You can try installing again and when it asks how you want to install specify "Do Something Else" and it will take you to the advanced partitioner. There, you should specify the partitions to use but do not click the box to format the /home partition if you have a separate one, or the / (root) partition if it also contains your /home.
The Ubiquity installer will then install over the top of the existing files leaving everything else in place. Because downgrading is not supported however, I'm not sure if this cause further breakage.
Regardless, I recommend that you back up all of your important data before you proceed any further. Development releases can and do get broken by an update at any time during the development cycle.
After you get it back up and running, make sure that you don't have update manager set to check for development releases. I'd even recommend that you set it to not tell you about stable releases either. They come out every six months - you can upgrade at any time if you really want to.