Obsessive Compulsive Disorder App For Teens

I am coding an IOS app to help teenagers with OCD. To help them cope with the thoughts. I am using Xcode to code. I want there to be a bottom navigation bar to get from screen to screen. Every code I tried, would create new screens instead of letting me switch between pre-existing ones. Or, it would mess up my screens. How do I change between already existing screens?