Yes you can. You just need to launch the installer, choose to update x-plane and untick the beta box. You will have a lot of clicks to perform, to accept replacing files by an older version, but eventually you will end up with X-plane 11.11r2, the latest stable version.
Laminar Research recommends to install beta versions on a second copy, to avoid this kind of inconvenient.
The latest beta version is 11.20b5, and it should be stable in a few days...