not all gear is upgradable
either check the wiki for your armor
or list it here, and i'm sure someone can help you.
and just to let you know.. i don't think it's worth upgrading 2 star or lower equipment (unless it has a good UV)
it's better to buy a 3 star from the AH
All you have to do is buy the upgraded gear's recipe then take the materials needed to the alchemy machine, pay the fee, and then your new gear is created.