This is what I did although only done once and not quite comfortable with it yet.
I downloaded and installed the iTunes App onto my PC.
I disconnected the wall end of my charging cable to expose the USB plug.
I plugged the cable into my iPhone and the USB end into a USB port on my PC.
I started the App on my PC.
It immediately recognized my 6S.
I won't remember all the steps as it's not connected now.
I tapped the Photos tab on the left side
I had created a folder on my iPhone and selected it (I thought). Remember this part as this was a slight issue.
I selected the folder on my PC holding the photos I wanted to upload.
I clicked Apply and saw a bar move across the top until the upload was completed.
The only two issues I had were slight.
1) The sort sequence of the files on my PC weren't the same as on the 6S. I can understand that as the file names were the original names from my camera and I'm not
sure how the iPhone handles 12355 vs 1233333. Which comes first and which should come first. Just to avoid that I'll put a prefix number before each name.
2) Even thought I "thought" I selected the destination folder on my 6S prior to uploading the files, all of the photos went into a "New" folder with the same name as the
source folder on my PC. It's possible I should have selected the files within the folder and not the folder.
Since I uploaded the photos using iPhones, I am not able to rename the destination folder. My plan is to delete the photos and folder using iTunes, add a prefix to the file names on my PC, create a new folder on the 6S, select it prior to uploading the photos carefully watch what I do and the result.
Hopefully, somebody will find this informative and useful.