This tutorial explains how to verify your Ubuntu 16.04.1 download manually.
Open a terminal by keyboard shortcut Ctrl + Alt + T and go to the Downloads directory to check your downloaded iso file.
Then run one of the the following commands from within the download directory.
Compare the hash (the alphanumeric string on left) that your machine calculated with the corresponding hash on the UbuntuHashes page: http://releases.ubuntu.com/16.04.1/MD5SUMS or http://releases.ubuntu.com/16.04/SHA256SUMS.
Open the page in your browser and copy the hash your machine calculated from the terminal into the “Find” box in your browser and search for the hash string. If there is a match the downloaded file is intact.