Go to your channel, select “customise channel” and move your mouse over your banner to find the pencil icon. Select “edit channel art”. You have the option to use images provided by YouTube or upload your own photos and channel art, designed using banner art templates and editing software such as Photoshop and InDesign. Once you’ve uploaded your banner, you’ll be able to view how it looks across all devices and adjust the crop where required. When you’re happy, select to apply your changes and you should see you channel art on your channel.