my iphone7 is displaying like a photo negative, how can I change it?

It would be better if you can please post a screenshot, in order to understand the phenomenon better. Images / pictures speak louder than words.

Triple click the home button and turn off, Classic Invert Colors or Smart Invert Colors if they are turned on.

Alternatively, go to Settings->Accessibility->Display & Text Size, and make sure Classic and Smart Invert colors are turned off.