Change Text on done orders

i want as an admin to change already done orders.

for example - the customer types a street name on a product, but with the wrong street nr. 

after the order is done i can load the product in the so called in german "bestellbetrachter" order viewer and edit the text but  

i can`t save this at the moment.

would be super update for customer service people who take a look at done orders!