You can see the update of 8.1in the windows store.... But all you have to do is update your windows 8. The essential file of store has to be upgraded to avail 8.1 update visible. Once you update windows 8. Restart your pc and then check your store.. 99% you should see windows 8.1 update.
The reason that your system doesnot meet requirement is because of 64 bit OS. I recommend to go with 32 bit. 8.1 does not need a lot hitech configuration. So you can install it.
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