The iPhone 6s is the first iPhone that can take 4K video, your iPhone's memory is going to get used up a lot quicker especially given that the 4K video files are almost 3 times that of 1080p videos that you could take with the iPhone 6. The SanDisk iXpand Flash Drive is a great solution to quickly move these 4K videos and photos from the iPhone and make room for more videos especially when you are away from that Mac or PC that you use to normally manage your media from.
Once you plug in the SanDisk iXpand into the Lightning connector of the iPhone 6s, you will be asked to go to the App Store
Select App Store, and you will get brought to the SanDisk iXpand Sync App download page on the App Store. Go ahead and select the download cloud icon and open the App once it's finished downloading.
The easiest way to move your media from the iPhone into the iXpand is to go to Camera Sync and turn on automatic sync. If you want to use the iXpand to keep on freeing up space from your iPhone, also select Delete from iPhone after Sync. When asked by the iPhone whether to allow the App to access your Photos, select OK.