@Magdalena
You can do it here (further down the page):
https://www.sky.com/help/articles/sky-mobile-upgrade
If you don't want a new sim then make sure you select the "I do not need a new plan/sim" option.
If you are struggling with this then give Sky a call on the free 150 number to do it.
If sending a phone back to Sky then make sure you get and keep proof of delivery.
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