had to work abroad and my sisteinlaw took it over. Now want to revert back to me.

[Weary+Traveller]

Opened my Sky account a long time ago but had to work abroad for quite a while so I had my daughter in law take on the account while I still paid for it.  She has subsequently moved home and I now need to change the account details to my original setup. I cannot upgrade or do anything without this and my dil has forgotten what password she used.

[Mark39]

Sky don't allow change of account owner, so if it was your account originally,  it still is.

Give Sky a call. If you don't know,the telephone password they'll be able to verify you by other means, if you request it.

[PandJ2020]

...and if your DIL has moved and taken any kit with her then you need it back.  (Although it belongs to Sky if Q)