It's possible to have multiple versions installed simultaneously, and in most cases the last version you installed will be the one that opens by default when double-clicking a file.
Since you have not told us your OS or how you are opening the files, there's not a lot more I can tell you. If you are opening the program from a shortcut of some sort, it sounds like it's pointing to the old version and you need another pointing to the new one.
Thank You very much for the answer. I didn't know that i can have both versions installed.
Now I see that I have actually more programmes dobble installed. Have a look at the picture:
Is it OK if I remove all the 2014 versjons? Are the 2015 versjons separete programs or are they working only together with the 2014 ones?
It takes so much space on the hard disk so I would like to remove all what is not needed.
It's OK to remove the 2014 versions (they're separate), but there isn't much reason to do so, and it may take some common files along, too. InDesign CC 2015 is still a little buggy, in my opinion, especially on Mac.