just move the SIM Card. If it is a different size you will have to by one and then you can just go to my tmobile.com and update the SIM there. If it is a larger sim (micro to nano) I have heard some have success with cutting it down, if it is smaller to larger (nano to micro) they do make adapters that will hold a nano in a micro space. If they are the same size however just swap it into the temporary device. If the temp device is not an iPhone make sure to disable iMessage or your text messages will get hijacked by Apple and not come to your other device. If it is another Apple product then just make sure to update the device in iMessage.
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