Why won't my iPhone 6s update?

Are you sure you have a 6S and not a 6? What version are you currently on?

Go to Settings > General > About and see which phone you have.

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.5

• If you have an iPhone 6S through 13, you can update to 15.5

Go to Settings > General > Software Update.

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

Update your iPhone, iPad, or iPod touch - Apple Support

Update using your computer -

Update iOS on iPhone - Apple Support

There is no such version as 13.5.2.

About iOS 13 Updates - Apple Support