Accidentally shut down mac while doing system updates

Hey. While updating I think I might have accidentally turned off my mid 2012 i7 MacBook Pro. A folder sign appears and there is a question mark on it. I google it to try and find a solution, and found that I have to go into recovery mode and select the boot drive. I did that and rebooted. Next thing I see is again, the folder with the question mark. I found more solutions online but all they said was to replace the hard drive data cable and this clearly isn't the case because I can see the hard drive in disk utilities. I did some more research and found that I can reinstall OS X from the recovery without losing data. Everything was fine until I got to the step where it asks me where to install the new operating system. I selected the only drive I had, the "Macintosh HD" hard drive and it said "You can't upgrade this version of OS X because a newer version is installed." What does this mean and how can I fix it without an external drive, without another Mac, and most importantly, without erasing any of my data. I'm pretty stressed out and don't know what to do. I also have some stuff that might be useful like a 16GB flash drive. I thought maybe I can install OS X on there and boot into it. Then I would transfer my files to another PC. Then I will format the mac's hard drive and install OS X to there again. And lastly, I will bring my files back from my PC to the Mac. I need to fix this ASAP because this is my dad's computer and I've messed it up. Thanks.