During the normal course of transactions Foreign currency accounts accounting values may not not match your book rate.
Doing a revaluation will set back the accounting value to match your book rate.
In the system you can use the Unrealised Exchange Module to do revaluation.
Adjustment -> Closing -> Unrealised Exchange.
Select the account to post the difference to.
under 'code'
select the accounts you want to reevaluate ( in my example it is the UOB USD account)
you can tick 'exclude currency' to prevent certain currency to be included in the revaluation (this is usefully when using Trade Debtor/Creditor accounts)
Click 'Post' Button.
*Do not use 'Post reverse' button if you have mistake in your exchange rate.
Adjust the rate and use the 'Post' Button.
'Post Reverse' Button will post an entry in on the first day of the next month ( 'Post Reverse' button is not a undo button)
Before doing the revaluation, you can generate the 'Unposted Unrealised exchange entries'
After the revaluation, you can generate the 'Posted Unrealised Exchange entries' to view the calculations.