I found my stolen iPhone what do I have to next

Wow! This is great.

Next step is to restore to factory defaults - Restore your iPhone, iPad, or iPod to factory settings - Apple Support - to completely purge it. You don't want to be using it without knowing if someone installed a malicious application there.

Hello ~ Take a look here:

I have found my lost iPhone; how do I rea… - Apple Community

If you filed a claim and the iPhone was replaced this will not apply.

~Karan-San~