How do I update my iPhone 6s Plus to iOS 13

You do not.

If indeed you have an iPhone 6S or 6S plus, you may upgrade it, but only to the current iOS version (14.2).

The iPhone 6S can be updated to iOS 14, but not iOS 13.

The iPhone 6 can not be updated beyond iOS 12.4.9.

No phone can be updated to iOS13 now. Any phone that could have been updated to 13 would instead update to 14. If you cannot get past 12 it’s probably because you have 6 Plus not a 6s Plus.