I need upgrade my iPhone 6s software and I can’t

First of all, check your model number in Settings>General>About>Model.

Model 6 or 6+ can support iOS 12.4.5

Model 6s or 6s+ can support iOS 13.4

That said, what happens when you try to update it? How are your trying to update it? Wirelessly or using a computer?

Start reading these instructions:--> Update your iPhone, iPad, or iPod touch – Apple Support

Regards

Giulio