I continued with the advanced startup, due to lack of information on that and genuine curiosity, and it did not give me an option to go back to my previous version of Windows. I also looked into System Restore which also did not offer anything of the sort. This means that the second option was a revert of the OS back to 8.1, which is what I am running now, and it is what was shown on the boot-thing, you mentioned. And only showed 8.1 there.
I must install Windows 10 again, but I hope that my files are still safe, as many of the programs I tried to run had missing shortcuts and I could not find them using the Start search (which, I admit is rather a useless and annoying feature of Windows 8.1 as it opens a search on the internet).
Anyway, thanks for the information you have provided.