You need to write to them at the address on this page. I strongly recommend you ignore the telephone and Whatsapp channels as these are deliberately setup to make cancelling as difficult as possible, with many reports of phoning and taking many hours, or clear cancellation instructions being completely ignored.
Send the letter recorded delivery, be clear on the end of service date you want (you have to include 30 days notice, and I'm assuming you're not in a fixed term contract). Know that VM customer service is so poor and unprofessional that you probably won't get any acknowledgement. Because billing is a month in arrears, you'll get one maybe two bills beyond the cancellation date (no they don't explain that) and possibly a refund that takes six weeks or so beyond that. It is then pot luck whether they will send you an equipment returns package. Even if they do send the returns package, keep the handover receipt when you send it back, there's a fair number of instances of VM insisting they've not had the kit back, and then being charged a non-return fee.
If there's any problems, you'll need to raise a written complaint, and when VM fob you off you'll need to escalate to the industry complaints adjudicator CISAS.
