Beta releases of Construct 2 come with the latest features, but they can also have bugs. Let's say you downloaded the new beta release with an annoying bug and saved your project using it. You want to open it in a stable release of Construct 2 (which doesn't have a bug in question), but projects cannot be opened in an older version of Construct 2 and you don't have any backups handy. Fortunately, there is an simple workaround.
1. Remove any features not available in a version of Construct 2 you want to revert to from your project (like newly introduced events or plugins) to avoid potential problems.
2. Save your project to a folder (choose Save As Project... from the menu).
3. Open your project's .caproj file in Notepad or any other text editor.
4. Locate this line:
<saved-with-version>some number</saved-with-version>
The number should be the release number followed by two zeroes (for example, if your project was saved in r116, the number will be 11600). For releases like r108.2 it might be slightly different.
5. Change this number to the number of release you want to revert to. Again, this should be the release number followed by two zeroes, so if you want to revert to r114, enter 11400.
6. Save the file. Now you should be able to open your project in an earlier version of Construct 2.