Revise return post assessment

Hello everyone,

i have filed my 2012-2013 taxes before time (31st july 2013). But i missed to declare some capital gain due to some issues. I want to revise my tax. Can i do it now? I found someone telling me that i cant revise tax under section 139(5) if my previous year return was approved in assessment by AO (i have already recieved an acknowlegement and some refund too). Can someone please guide on the process?

Also for a foreign stock thats not listed in india, is their a difference between long term and short term gain? Or i have to pay flat 20% on capital gain?

thanks

sunil