Access the accounts menu by either clicking on 'Current Balance' or clicking on 'accounts' on the left tab menu.
After that, press the three dots icon of the account you wish to edit for more options and then press 'adjust balance'.
After that, the adjustment menu will appear and you will be able to select the balance and if you wish it is adjusted using a new transaction (To adjust your balance, an adjustment transaction will be created) or if you want the initial balance to be changed.
After that, press 'Save' in order to save your adjustment and it's done! =)