Yes, you can edit the information listed in the split gift grid when you edit a gift. Click on the amount from the Gifts grid or the constituent's Gifts tab to open the transaction record. 

If you are changing the split amount, but not the total, you can click on Amount in the Split Gift section. Be sure that the total percentage is 100% and the total amount matches the Amount at the top of the record. 

If you are changing the total Amount of the gift, be sure to enter the correct total Amount at the top of the transaction and make the appropriate splits with the total percentage of 100%.

Click the green Save button at the top or bottom of the page to save the changes.

Once you save the changes, you will see the updated data on all related grids.