When you log into your My T-Mobile account, you can check your upgrade availability per device line you have. If there is no "UPGRADE" button next to your device, then you are not currently eligible for an upgrade.
You can also read more about this HERE.
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You can also contact customer service and ask them.
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With no contracts, you can basically upgrade whenever you want. (Credit score might change a down payment amount) JUMP plans are a little different.