Ubuntu 15.04 is obsolete and unsupported: https://lists.ubuntu.com/archives/ub...ry/000203.html
You can download the current release (16.04.1 at the time of this writing) here: http://www.ubuntu.com/download/desktop
You'll notice that page has instructions how to create a live USB whether you are currently using Linux, Windows, or OSX. It is also worth reading the instructions how to verify your download. You can even buy a USB stick with Ubuntu already on it, if you like!
I agree with jefro's advice in your other thread, that you should purchase a second, inexpensive flash drive for Linux (2gb is all you need), so that you don't risk the data on your current device.