How to set compiler MinGw?
By default i have MSVC in my Qt app
This most probably means you've installed the MSVC version of Qt libraries. If you want to use the mingw version, then you must download the mingw version of Qt.
Edit: there is no way to mix both version.
to add to @JohanSolo
The easiest way on windows to start is using the online installer, choosing the newest MinGW version and the required newest MinGW compiler suite. All is accessible directly in online installer and you start away from compatibility issues.