If you have a Microsoft Lumia 950XL and you wish to clear it of all its contents then you will need to do a hard reset of the phone which will delete all the content on the phone and set the mobile phone back to to its factory settings. You can backup your content by using many of the programs out there.
The Lumia range of mobile phone all use the same settings for a soft reset as well as the hard reset. You can see how to do it by looking at the article on the Microsoft Lumia which has been listed on this website. The procedure for the Lumia 950 XL Dual Sim for the reset is the exact same.