Unfortunately this exploit has been fixed within a day of it being found.
This is a bot to help people with using the new discord banners without having a verified server, the feautre is coming to normal servers soon, however this takes advantage of an exploit to use it sooner.
First of all, make sure you have a bot made ready in https://discordapp.com/developers/applications/ so that you can quickly grab the token and get started as soon as possible.
Download the zip of this repo and extract, run npm install like you would any other NodeJS application, but then this is where it gets different. Go into the node_modules folder follow this path discord.js/src/structures/ and drop in the file located in the exploit folder, this should replace an already present file. Then run it as normal (npm start).
You should now be able to use the command !setbanner <bannerurl> where the <bannerurl> part is a url of the image you would like.