Posso aggiungere dati sulla giornata di ieri? Can I adding data in yesterday?

Can I correct my yesterday day with adding data, if yesterday I forgot to insert something?

Yes, you can. For example you can edit activities, weight data, calories, water intake, and many other data fields. If you ask where, then it depends what exactly you want to edit, but it is practically aways on the respective screen that you want to change. Often accessible through the edit function in the menu (on some screens in the upper right corner, but sometimes it can be elsewhere). The web interface of Garmin Connect may give even more possibilities than the phone app, so you may want to check it out too.