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DiskAid can easily copy your contacts from your device, but you need to use iTunes to transfer them back. Once you have successfully created your VCF file/s (standard format for contacts), you need to import them into a contact manager application, such as Contacts or Address Book on a Mac, or alternatively Windows Contacts or MS Outlook on a PC. Depending upon the contact manager you are using, you may need to have initially exported your contacts as individual VCF files (MS Outlook for example does not support multiple contacts on a single VCF file)
Once you have imported your contacts into your contact manager, you need to direct iTunes to Sync contacts from this applications database. To do so, open iTunes, then plug in your device and select it from the left navigation panel. This will open up your Sync options in the iTunes main window. If you click on the "Info" tab you can find the options for Syncing Contacts. Here you can select which contact manager to Sync from and also if you wish to Sync your entire contacts list or just specific groups that you have created.