Within the Settings tab you will find a setting for Default Accounts: from here you can select the default accounts for Retained Earnings and Tax:
Retained Earnings - The current period earnings balance (all P&L account balances) will flow into this account in the next period. It must be an account that falls under a Equity or Reserve classification. Note: If you change this account in any period, that change will be reflected in all prior periods also - be careful if you choose to change this!
Tax Account Settings - Accounts designated for the Tax on Sales and/or Expenses. It is possible to change the designated account after entries have been posted, but this will not change the account set in the existing entries already made (you would have to change these individually)