How do I update my iPhone 6s with latest version?

Your stats say you have an iPhone 6 running iOS 12, I don't think you have a 6S.

An iPhone 6/6plus can't be updated beyond 12.5.6, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

If you do have a 6S,

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.6

• If you have an iPhone 6S through 7, you can update to 15.7

• If you have an iPhone 8 or higher, you can update to 16AC.0.3

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

Update using your computer -

Update iOS on iPhone - Apple Support