We appreciate your interest in adding a line! The device would be able to be cancelled at any time without penalty as long as no contract or device payment currently exist. May I ask what software version you're currently running on the device?
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What plan are you on? If it's a shared plan like More Everything or the Verizon plan, then you can add the tablet for $10/month; add it at the start of a billing cycle and leave it on for the full month to get full advantage of the shared data.
The same would apply if it 's on an older legacy plan (add at the beginning o fhte billing cycle and leave it on for the full month) - adding it for a short period would result in a pro-rated amount of data which may not be sufficient to complete the update (2 GB plan, active for one week translates to ~500MB data...)