If you choose 'Yes' here it will update all 'trusty' to 'xenial' entries. In other cases, you may be asked if you wish to replace a configuration file that you have modified with the default version from the package that is being installed.
This is often a judgment call, and is likely to require knowledge about specific software that is outside the scope of this tutorial.
This guide will explain the process for systems including (but not limited to) Digital Ocean Droplets running Ubuntu 15.10.
Comprehensive backups and extensive testing are strongly advised.For other systems under your control, remember that it's safest to perform major operating system upgrades only when you have direct physical access to the machine.At the prompt, type y and press Enter to continue: Reading cache Checking package manager Continue running under SSH? It is not recommended to perform a upgrade over ssh currently because in case of failure it is harder to recover.The best way to accomplish this is to make a backup of your entire filesystem.Failing that, ensure that you have copies of user home directories, any custom configuration files, and data stored by services such as relational databases.