To use Dynamic Type, go to your phone's Settings and select General. Then choose the Accessibility option, and select Larger Text. Use the slider at the bottom of the screen to increase the font size in for your apps.
Note: You do not need an iPhone X to take advantage of the larger font size. This update supports the feature on all iPhones running iOS 10 or later. Also note that only the first level of larger font is supported; you do not need to turn on Larger Accessibility Sizes.