When you create a balance sheet account like Bank or Credit Card in Chart of Accounts, you can specify an Opening Balance for that account. However, you cannot change it after you have created the account as this field is not visible when you open the account details.
As a workaround, you can go into the register of the account by clicking Use Register against it in the Chart of Accounts list. In the register, you can see that setting the Opening Balance creates a transaction involving the Opening Balance Equity - 3000 account with a Payment (for credit card accounts) or Deposit (for bank accounts) equal to the Opening Balance. To change the effective Opening Balance, you should delete this transaction and then recreate it with the new value for Opening Balance in Payment or Deposit. Check CORE Help Center for details on using Registers.