How to present a view controller using the SWIFT iOS tab bar controller

Question

I want the view controller to be modified by clicking the tab bar button.

I was recommended to use the tab bar delegate features

Where to place delegate functions in the tab bar? In the AppDelegate file?

And what a pleasure I would use for the actual view controller view. So instead of having the tab bar switch the main view to the view controller, I would like it to represent the view controller instead.