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This question already has an answer here:

I am unable to update through software updater. After clicking install now, it shows "requires installation from untrusted package".

Then there is two options: Settings and OK. I click OK but the software updater window closes and stops.

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marked as duplicate by karel, Aditya, Mitch Mar 13 '14 at 7:33

This question was marked as an exact duplicate of an existing question.

    
This is irritating i cant post screenshots – user204983 Mar 13 '14 at 4:10
    
you can't able to post a image.please post the image link. – Avinash Raj Mar 13 '14 at 4:10
    
@user204983 upload it into imgur.com and paste the link into your question editing it. You can try to solve the problem following this procedure. Tell us if that doesn't work – Lucio Mar 13 '14 at 4:25
    
Looks like key error issue. Some repository site's gpg key is missing maybe. run in terminal (sudo apt-get update), post the output(last section/errors). – shantanu Mar 13 '14 at 4:43

might be useful to look at your sources file you can get it like this:

cat /etc/apt/sources.list

but you might try this:

sudo apt-get clean
cd /var/lib/apt
sudo mv lists lists.old
sudo mkdir -p lists/partial
sudo apt-get clean 
sudo apt-get update

source

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