On the page Organization settings -> Accounts, when attempting to edit a sub-account the edit button is greyed out as shown below.
1. The user attempting to edit the sub-account is not an Org Admin. The Org Admin role is required for a user to edit sub-accounts.
2. When clicking on the accounts button, you will see the Organization Account listed first and the primary sub-account listed directly below it. In the example below the primary sub-account and Organization are LWCS.
The primary sub-account is the sub-account with the same name as the Organization.
The primary sub-account can NOT be edited in any way by the end-user. You can NOT edit the account name OR the account description, nor can you change the account owner.
If you need to make changes to the primary sub-account please open a support ticket via the support portal.