@Irsanaz
You should be able to do this via the MySky app, by calling Sky on 150 or using this link, half way down the page:
https://www.sky.com/help/articles/sky-mobile-upgrade
Do not you phone that you will be returning to Sky still has to be paid for so if the value Sky gives you for it is not enough then the difference will be added to your next available bill after the swap has been completed.
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