If you want to enhance your weapon to a higher level, please click from “The Home screen” -> Weapon (on the bottom left) -> “Enhance”, you will see how to do it. Enhancing a weapon requires hammers or other weapons as building materials, and Silver coins.
If you want to evolve your weapon to the next star level, you need to enhance your weapon to the max level first, and you also need one or more of the same weapon at any level as the evolving material. Note, you cannot use a weapon with “+1” or “+2” as material for weapons evolution.
You can only evolve your weapon to the next star level weapon of the same element. You can see which weapons are of the same element by tapping “The Home screen” -> “Weapon” -> “Vault” -> “Weapon Info” (on upper right).