In case you do not count on enough space on the device to perform the update via OTA or if you prefer to use a "safer" method, it is best to upgrade to iOS 8.3 via PC, Mac or Windows, using iTunes.
This process is quite simple, so they just have to:
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Connect the iPhone or iPad to your computer.
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Open iTunes.
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In iTunes select the Summary tab of the device you're upgrading.
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If you notice of availability of the new iOS version 8.3 does not appear, click on Check for updates.
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At this point we a popup window appears, select Download and update here.
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Now a list will appear with the new features included in iOS 8.3, click Next.
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Finally, in the terms and conditions, click on Accept.
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Is likely to continue the iTunes update process required to enter the lock code of the device.
When we have done those steps the software is downloaded and installed. After completing this procedure will have to follow the instructions on the screen of the device, as introducing our Apple ID password and then you can begin using the updated iOS 8.3 device.