If you're getting Universal Credit and your circumstances change (for example, if a partner or dependent child moves in with you, you start work, or you move house) you should report this as soon as possible to the Department for Work and Pensions on:
Phone: 0345 600 0723
Textphone: 0345 600 0743
If your circumstances change and you don't tell someone, your benefits may be overpaid (and you'll have to pay them back) or underpaid (and you may struggle to make ends meet). It could also be seen as benefit fraud.