18 i still in beta-testing phase meaning that you should not use it for critical work. A beta test version may still contain bugs, some functions will not work fully, it might even fail in your project and so on. The update is additionally sort of one-way for your projects, it is possible if you prepare for it to go back to 17 but not totally easy. The update function in DR will suggest 18 once it is fully ready. Meanwhile it will update to 17 versions.
With the warnings given above -- if you still want to test drive 18 you need to download it specifically. Check the support page:
https://www.blackmagicdesign.com/suppor ... and-fusion