If you want to reset the Carryover Balance or if your Carryover Balance is not correct for one of your Funds then you will add a transaction to make up for the difference, so for example if the Carryover Balance is $100 less than it should be then you would need to add an income (+) transaction for $100 to the Fund to correct that. If the Carryover Balance is $100 more than it should be then you would need to add an expense (-) transaction for -$100 to the Fund to correct it.
The steps to manually add a transaction are outlined for you in this article:
How to Manually Create and Track Transactions