There used to be plugins that could change your name and skin, but unfortunaly those plugins are broken, so theres a more technical way of doing this.
For changing your name, install Essentials. Then do /Nick <IGN> <New Name>, (This will not change the name above you... i think... im not sure...) To get rid of the ~, go to File Manager at minehut.com > Plugins > Essentials > Config.yml. Once finding the right option, changing it to "", and typing /essentials reload in-game, you should be good.
For changing your skin, should be easier. Click Profile | Minecraft, or go to the minecraft launcher. Find and download the skin you want, and apply it.