I'm assuming the phones are on different networks. You would contact the service provider of the old phone and tell them you want to 'migrate' the number to a different network. You are asking for the 'Port Authorisation Code' (PAC) - they should give you this for free. You give the PAC to the new provider and they'll do the change for you.
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You will have to go to your service provider and ask them this question they will be able to do in no time with no problem.