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There are actually a few different ways to do this and it depends on how the phone was set up and if it was synced with a computer or icloud.
The device will usually need to be reset and then you will have to restore from a back up on itunes or icloud.
What would probably be easiest for you is to look at the step by step instructions on the link I am going to give you to see which will be your best way to go about this since you or your so would know best how the phone set up and backed up.
All of the options are explained in detail here
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