You can set base permissions that apply to all members of an organization when accessing any of the organization's repositories. Base permissions do not apply to outside collaborators.
By default, members of an organization will have Read permissions to the organization's repositories.
If someone with admin access to an organization's repository grants a member a higher level of access for the repository, the higher level of access overrides the base permission.
If you've created a custom repository role with an inherited role that is lower access than your organization's base permissions, any members assigned to that role will default to the organization's base permissions rather than the inherited role. For more information, see "Managing custom repository roles for an organization."
In the top right corner of GitHub.com, click your profile photo, then click Your organizations.
Next to the organization, click Settings.
In the "Access" section of the sidebar, click Member privileges.
Under "Base permissions", use the drop-down to select new base permissions.
Review the changes. To confirm, click Change default permission to PERMISSION.