you have to write a surrender letter. you need to make copies of this letter. one letter you give to the storeroom along with modem and everything that came along with it. if something is missing, they will make you state it in the letter and there is a fine.
other letter you need to submit to the area in charge. the guys in storeroom helped me in writing this letter so just ask at the exchange.
I did this some 2 years back so little sketchy on details.