So I have iPhone 5c ME561LP/A with demo content. Can I replace the demo content with fully functional iOS?
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Probably, all you should have to do is wipe and restore it to the latest iOS version via iTunes:
Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings
A factory restore erases the information and settings on your iPhone, iPad, or iPod and installs the latest iOS or iPod software.