How do update to os x mavericks

I believe os mavericks is available in the uk for free, i have an iMac 21.5 inch late 2009 software OS Lion 10.7.5 (11G63) processor 3.06 dual core, but i can not get into my app store to download it, I can get iTunes no problem but can not download Mavericks, can some one please help. Thank You


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