When an allocation has been posted, you will not be able to change the Fund. You need to use the "Apply Adjustment" button if you would like to re-allocate to a different Fund.
In the allocation record, click on the "Apply Adjustment" button found on top (if you do not see the Apply Adjustment button, you need to check your configuration and make sure that this button is enabled).
Start typing into the box and the fund selection will start to appear :
Once you've selected which fund, click submit and you will be taken back to the Allocation Record where you can see that the adjustment has been made:
When you click View All you can see more details about the adjustments being made:
You may now include these new allocations the next time you post to finance.