Open Settings > Organization. This area requires Full Access and Manage Settings.
Organization Logo supplies the general business logo. Square Logo (App Branding) supplies a square asset for compact application branding. Use a clear image in the shape requested by the upload control; inspect the preview before saving.
The separate login-branding section controls the branded sign-in experience. Its enable switch determines whether the custom branding is used.
Public forms also have their own branding controls, so check the form editor if a form should look different. Branding changes appearance; it does not change membership, permissions, MFA, or access groups.