distribution upgrade from ubuntu 14.04 to 16.04

[sifalios]

Hi I have an Ubuntu 14 VM in Digital Ocean with VestaCP installed and I tried to upgrade the OS to Ubuntu 16 but I got the following error:
Error during update

A problem occurred during the update. This is usually some sort of
network problem, please check your network connection and retry.

W:Failed to fetch
http://apt.vestacp.com/trusty/dists/xen ... 4/Packages
404 Not Found
, W:Failed to fetch
http://apt.vestacp.com/trusty/dists/xen ... 6/Packages
404 Not Found
, E:Some index files failed to download. They have been ignored, or
old ones used instead.

Are there any suggestion?

Thank you in advance

Iosif Koidis