I want to upgrade my software am using iPhone 7

See instructions in Update your iPhone, iPad, or iPod touch - Apple Support

I want to upgrade my iPhone software and am using iPhone 6s plus please help me please.

Follow the steps in the blue link provided by FoxFifth above your own post.

If your iPhone is running iOs 12.5.4, then its not an iPhone 6s. Its an iPhone 6, and cannot be updated beyond iOs 12.5.4