It is generally a good idea to get rid of old transactions, once you do not need them any longer. This will free up some space on your device, and most importantly, will speed up the working speed of FreeCoins. The more transactions the program has to keep track of, the slower it will move. Purging transactions will not affect your current balances, it will simply remove transactions older then a certain date.
Another thing you may find useful is to periodically reset the balance of the Income and Expense accounts to 0. Suppose that you do this at the start of each month. By the end of the month you will have a clear picture of how much you earned by looking at your Income accounts, how much you spent and what you spent money on by looking at your Expense accounts.
Should you wish to modify the balance of an account, you have several ways of doing this. The best way, is to create a transaction from that account to an equity account for the amount you need to adjust the balance by. Otherwise you can just adjust the balance by opening the `account details' dialog and simply entering a new balance.
There are another two tools you can use. One of them is the `Adjust balance' menu entry. This will prompt you for a new balance and enter a new transaction to bring the account to the new balance. If for some reason the cleared and uncleared balance of an account get out of sync, you can adjust the cleared balance only by using the `Adjust cleared balance' menu entry. You can get to these entries by opening the transaction ledger for the account you wish to manipulate and then opening up the `Account' menu.