set old Iphone5 for new user and new phone number

First follow these steps: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Then, create a new Apple ID for the new user. How to create a new Apple ID - Apple Support

Then, Set up your iPhone, iPad, or iPod touch - Apple Support

Verify with your cellphone carrier company, how you can use the iPhone 5 with a new cell number (They may have to unlock the phone so that it will accept a new number)