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I'm new in Ubuntu 16 and want to know, how to fix this problem.
Reading package lists... Done Building dependency tree Reading state information... Done 4 packages can be upgraded. Run 'apt list --upgradable' to see them. W: http://archive.canonical.com/dists/precise/Release.gpg: Signature by key 630239CC130E1A7FD81A27B140976EAF437D05B5 uses weak digest algorithm (SHA1)