Zara won’t exchange for another size

Hi , I bought a pants from Zara on the 18 of September which is 2 days ago my slip was lost I went in today which is the 20 September to exchange for a bigger size they couldn’t exchange with a slip , their explanation was because of the barcode all tags intact on the pants , if it was big I could alter it but small , here I am not knowing who can help me , please can you assist, I’m so help less , thank You