I have an iPhone 6s and I would like to know how to get the most current iOS possible

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information.

Hello ~ Take a look here:

Identify your iPhone model - Apple Support

If your iPhone, iPad, or iPod touch won't update - Apple Support

~Katana-San~

Use a computer or the Software Update function in Settings to update it to 15.5.

(221545)

Are you sure you have a 6s rather than a 6 or 6 plus?

What is the existing version of iOS?

What happens when you attempt to update?

Any error messages?

Stormyduck wrote:

I have a 6s though and it won’t allow me to update to the 15.5

What happens when you try?

If it's running iOS 12.5.5 it is not a 6s.