its very simple :
open your whatsapp and go to settings menu like this
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choose option " settings " like this
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choose option " chats" like this
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choose option " chat backup" like this
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now choose an option of light green icon " BACKUP" like this
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now it will ask you
In which account you want to save or backup your chats?
select any option like this
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thats it. thankyou....