change font size in macbook air

I surmise you are seeking to increase the size of all the text you see in all applications system-wide. In that case please open System Preferences > Accessibility. Review the options available to you under Vision. You can "zoom" the entire screen using a key combination.

Some applications (such as Safari) allow you to zoom in on the display using a two-finger trackpad gesture.

Font sizes themselves are changed in text editing applications (such as Pages) using a menu selection, but there is no equivalent way of doing that in the Finder (for example) or other apps that are not intended to manipulate text.

I dont have Accessibility.

But I worked out that now I just use Command > +or - to increase or reduce font size of my screen.