Depends on your service provider. I know MTN allows you to upgrade on the 21st month of your contract.
Do determine when your next upgrade will be, use the service provider options below.
Vodacom:
Simply SMS ‘UG’ to 31050 from your Vodacom cellphone.
Mobisite: m.vodacom.co.za/upgrades
Upgrades Call Centre: 082 1959
Vodacom approved Outlet: Visit any Vodacom approved outlet