First backup your project just in case something happens then open your .caproj in a text editor
locate this line <saved-with-version>18800</saved-with-version>
and change in Note 18800 to 18400 and that will make it able to open in the last stable version, but if you've added anything that was made in the betas it can break your project so make sure you backup. If you want to downgrade to a different version you just have to make the number that Build version with two zeros added.