Software updater warning: requires installation of untrusted packages
This is some problem in yout cache run those commands to solve it:
sudo apt-get clean
sudo mv /var/lib/apt/lists /var/lib/apt/lists.bak
sudo mkdir -p /var/lib/apt/lists/partial
sudo apt-get clean
sudo apt-get update
source and more info
I had the same problem. This happens due to the missing of some GPG keys.
Go to applications->accessories->terminal or press CTRL+ALT+T.
And, type below commands:-
sudo apt-get update --fix-missing
You will get an error saying that some public key is not available. Note the key string ( after the NO_PUBKEY; eg:3C962022012520A0 )
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com XXXXXX
Replace the XXXXXX with the key string
Ex:- sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 3C962022012520A0
Now try updating your system again.
sudo apt-get update
This will surely solve this issue.