Then , a popup will show and then click on Branding.
After you have clicked , another popup box will appear like below and now you have to upload your custom footer image or subscriber button in any format like jpeg or png. See below for reference.
In my channel , I already inserted subscriber button in my channel so for new vlogger , one should have to click on remove or replace for adding a subscriber button . Watch Video also for better understanding. Download Subscriber Button