How can I fix these errors?
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W: GPG error: http://dl.google.com/linux/earth/deb stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4EB27DB2A3B88B8B
E: The repository 'http://dl.google.com/linux/earth/deb stable InRelease' is not signed.
W: GPG error: http://archive.canonical.com precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5
E: The repository 'http://archive.canonical.com precise Release' is not signed.
W: GPG error: https://repo.skype.com/deb stable InRelease: The following signatures were invalid: EXPKEYSIG 1F3045A5DF7587C3 Skype Linux Client Repository <se-um@microsoft.com>
E: The repository 'https://repo.skype.com/deb stable InRelease' is not signed.
Error - Return code: 100