I can't open Blender 2.8 files after I save them. Because it tries to open with 2.79 Blender 2.79b is installed (from .exe) on my computer, currently. And I just downloaded the Blender 2.8 release candidate 2 (.zip).

When I create something in Blender 2.8, after I save it, I cannot open it again because the computer tries to open the blend file in version 2.79b. And manually selecting the 2.8 version from zip file does not work.

How can I open my blender 2.8 files? Do I have to delete Blender 2.79b?
You don't need to delete anything.
Just open blender 2.8, in the top menu select File > Open and open the file created in 2.8

If you want to permanently open all *.blend files with 2.8 you would need to modify your operating system's default applications (do a search online for that. Customizing the operating system is not a topic for this site).