If the Volume Button still works, then you simply have to replace the Shiny Silver cover to the button. If this is the case, buy the external button (costs about $1). Peel the Shiny Silver cover off the black portion beneath. Then just glue the Shiny Silver cover onto your iPhone. If the Volume Button is completely missing and does not function, this is a very difficult repair, including removing the metal bezel ring from the iPhone back case so you can install outer portion of Volume button. The inner Volume button is part of the Headphone Jack Flex cable #5, which in itself is one of the most difficult repairs of all iPhone repairs.