When your account is connected with more than one Organisation, the Default Login Organisation is the landing Organisation when you sign in to your account.
To set or change your Default Login Organisation, navigate to Account > My Settings
Under the Login Profile Set Details, click Default Login Organisation
Under Default Login Organisation, click the Edit (Pencil icon) next to the current Default Login Organisation
Select from the list of organisations connected to your account, then click Save to set the new Default Login Organisation