Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
I'm very sorry that you lost your files. Unfortunately you can't undo it. It is almost impossible to recover overwritten data from the iPhone.
Your only hope is iCloud backup. If there is iCloud backup of new phone, then you can restore your phone to that backup.
Now you can erase your iPhone and restore to iCloud backup if any available. Go to Settings > General > Reset and tap Erase All Contents and Settings. After erasing the iPhone, it will ask you to setup it. While setting up, select Restore from iCloud and this will restore your iPhone to more recent backup of this phone (not old phone). You can also setup it as new iPhone if you want after erasing all contents and settings.
Let me know how it goes please.