This question might have been answered somewhere but have not been able to find it. So, I have a Mid-2009 MacBook Pro where I am still running OS X Yosemite since I forgot to update to OS X El Capitan when it was on the App Store before they changed it to macOS Sierra.
If I try to install macOS Sierra through the App Store it doesn't let me because it says that:
"This version of macOS 10.12 cannot be installed on this computer."
I started looking to see if I can find OS X El Capitan but there is no result through the Mac App Store nor Apple Developers site. So my last resort was through Google, where I found several options on where to download OS X El Capitan version
So, my main question is if I download from these third party websites, can I install on top of my current OS like it is done through the App Store or do I have to do a clean install?
Would appreciate any help.