You'll need to call Customer Services to do that - it's 789 from your mobile, or 0345 454 1111 from any other phone.
If you joined Virgin Mobile before December 2017 and haven't had a new SIM after that, you'll be sent a new SIM as part of the number change and your number won't change until you've put the new SIM in your mobile.
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I work for Virgin Media - but all opinions posted here are my own