When you had clicked to install CC 2017, a pop up like below must have appeared...
Selecting the option in blue would install CC 2017 over CC 2015. Selecting the option in red would have installed CC 2017 without overriding CC 2015.
Now, what you can do is to install CC 2015 again. This way you would get both CC 2017(installed previously which deleted your CC 2015) and the newly installed CC 2015 (will be visible in Previous version section of creative cloud applications)
Thanks so much already for your answer but I'm not there yet ...
Where do I find CC 2015 again? Because when I search for It and click on install the program always wants to open CC 2017?
If you uninstalled CC 2015 during installation, you need to reinstall it, as amaarora advised.
If you chose "keep previous version" under Advanced Options, you can create a shortcut and put it on the dock and if desired, make 2015 the default application to open with double-clicking. On a Mac it's in Get Info. On Windows, it's Open With.
Thank you very much, problem is solved!