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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?