Cannot upgrade mac mini
Look up your model here.

http://www.everymac.com/systems/apple/mac_mini/index-macmini.html

Then, scroll down to the section marked "Maximum MacOS:", it's towards the bottom on the right.

One of my Mac Minis is the early 2009 Core2Duo model, which is the oldest Mini which supports Yosemite.  If you have a model which is older than 2009, it will only support Lion (Core2Duo), Snow Leopard (CoreDuo/CoreSolo), or Leopard (PowerPC).