Refer: https://askubuntu.com/questions/848085/upgrade-from-15-10-to-16-04-no-new-release-found
You can change your software sources from wily to xenial, then update, upgrade and dist-upgrade.
1. 'sudo nano /etc/apt/sources.list'
2. Check every rows to find the word wily and change those to xenial 2. Ctrl-X to save, Y to yes, Press Enter key
3. Update, upgrade, dist-upgrade. – Arijit Chatterjee