Thanks for reaching out. Just to check, are you seeing an error message when you're trying to change channel banner? Also, please check our Help Center article which covers different scenarios in this regard: https://goo.gl/2gq4tQ . Keep us posted.
Thanks for the info. Just to confirm, have you tried using a different device/browser? Also, if you're using a desktop, try clearing the browser's cache & cookies and use Incognito mode to see if it helps. Keep us posted.