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Yes. You can upgrade the RAM. . You can buy RAM memory from here --> https://eshop.macsales.com/shop/apple/memory?_ga=1.266074895.1071240923.1473193695
Here are the instructions --> https://support.apple.com/en-us/HT201191
If you have any questions or would like any additional information, please don't hesitate to ask
Note : Even the official website says iMac 2011 support 16GB memory maximum, the 27" 2011 iMac can unofficially support up to 32GB.