1. Open the account you need to set as the default one, pick the ID in the URL and copy it :
2. Go to (DEBUG) > Technical > Company Properties
3. To change the default account for suppliers, open property_account_payable_id
4. Replace the ID with the correct one (paste it)
5. To change the default account for customer, open property_account_receivable_id
6. Replace the ID with the correct one (paste it)