After you've created your custom branding, you can go back and change anything you want.
To edit your custom branding
Sign in to the Azure portal using a Global administrator account for the directory.
Select Azure Active Directory, and then select Company branding, and then select Configure.
On the Configure company branding page, add, remove, or change any of the information, based on the descriptions in the Customize your Azure AD sign-in page section of this article.
It can take up to an hour for any changes you made to the sign-in page branding to appear.