How can I upgrade an old iMac from 10.6.8? Model: iMac11,2 (Core i3-550, MC509LL/A)


It was shipped with Snow Leopard 10.6.4, and I managed to update to 10.6.8 via Apple Software Update. After some research I found that this old iMac is capable of running Sierra (10.12.x) and made a few attempts to upgrade.

I noticed that I must first upgrade it to Lion (10.7.5) or El Capitan (10.11.6), both of which are no longer available from the Mac App Store. 

But, all info I found from Google tells me to upgrade via MAS. (That's why I'm adding a month in the title.) Any ideas?
You have a couple of options you can try:


Download  El Capitan from the App Store
Take your Mac into an Apple Store where they can download and install El Capitan for you.