I don't think you can once the account has been set up. The way I do it is to reverse the OB (using the original date) to nominal 3999, then using the same date, put in the amount you want it to be. Make sure you put something meaningful in the description to explain the change. To do this use Transactions>Manual Journal Entry.
Before you do this, because EA cannot open different functions windows at the same time, make a note of the Debit and Credit so that you don't post the wrong way round and have to make further correcting entries.