I would like to modify element like UIAlertView, UIButton or UITextField, but I can't find information about how to do it using PaintCode. Any advice?

Check out Apple's documentation for the UI element you want to modify, particularly the methods that are there to allow style customization.

Some of these objects allow subclassing and overriding its drawing methods (so you can literally draw anything in place of original design). other objects have methods available specifically for customization of the object visual style. UIButton is the typical object for the latter case: it implemenets methods like setBackgroundImage:forState: or Swift setBackgroundImage(_:forState:), which can be used to customize UIButton's background image.

Please, check Does PaintCode generate images or drawing code for UI control states (like button normal, pressed, and hover states)? and How can I use PaintCode to apply a different “style” to a button or other control while using the same event handling code? for more information.