Changing commissions after sales are made

hi there, 

Having made a hash of an affiliate store's commission settings it is showing an amount owing to the store owner which WAYYYYYY too high. 

I can work out the correct amount with a spreadsheet of the orders, but is there a way to change the amount each sale has made AFTER the sale has been made and completed? 

Ideally I would like to go in and edit it so that moving forwards everything correct and the customer just sees the correct totals if they look at their store backend. 

But I can't see a way to posthumously alter commissions. 

Thanks, 

Evan