You can find instructions for restoring your Apple device to factory settings here:

https://support.apple.com/en-us/HT201252 

Before sending your Apple device back to Komplett, please ensure that the device had been removed from iCloud / Find my iphone. You can find instructions here: 

https://support.apple.com/kb/PH2702?locale=nl_NL&viewlocale=en_GB