Before you proceed, I warn you that its not wise to upgrade to Ubuntu 15.04 from Ubuntu 14.04 (LTS). Ubuntu 14.04 is a LTS which has longer supports compared to Ubuntu 15.04 (Normal release). But here I just want to show you that we can easily upgrade to Ubuntu 15.04 from Ubuntu 14.04. I am using Ubuntu 14.04.2 Server edition.
This is my Ubuntu Server 14.04 Trusty Tahr
To upgrade to Ubuntu 15.04 Vivid Vervet, simply open Terminal or we can remotely connect to Ubuntu Server via SSH. The second way is not recommended by Ubuntu official. Anyway, lets get started. This method will first update/upgrade Ubuntu 14.04 to Ubuntu 14.10.
1. First we need to modify the upgrade restriction. Type this command and change Prompt=lts to Prompt=normal
sudo pico /etc/update-manager/release-upgrades
2. Install update-manager-core if you don’t have it already
sudo apt-get install update-manager-core
3. Launch the upgrade tool
Wait until the process is finished. It will download files from the internet. When finished, the system will be restarted. After restart you should run the new Ubuntu 14.10.
Now we can upgrade to Ubuntu 15.04 from Ubuntu 14.10 by executing the command
Again, wait until it download files and install them.