The owners iPhone is now mine yet when I open the Apple icon to order anything his email keeps popping up how do I change it to mine. I don’t know his password and cannot use

Check the App Store and iTunes Store to see if there are any updates for apps/purchases. A request for an old Apple ID password is often caused by updates to apps/purchases that were made when using the old ID. Those purchases are tied to the old ID and can't be transferred. One option is to delete the apps/purchases made by the old ID and download them using the new ID if desired.

The previous owner should have wiped the iPhone clean, so you could set it up from scratch,

If they did not do this its possible the iPhone may lock with activation lock and be unusable.

The first thing to check when acquiring a new iPhone is whether or not the previous owner's ID is still signed in to anything. It should not be.

Before doing anything else, check if there is an Apple ID different to your won signed in for iCloud in Settings->Account(top of settings) and verify what Apple ID is signed in for iCloud there

If there is the previous owner's Apple ID there, you will not be able to remove it without its password, and the iPhone can potentially be locked by the previous owner at any moment, rendering it unusable.

You will need to contact the previous owner and have them follow the steps here: What to do before you sell or give away your iPhone, iPad, or iPod touch - Apple Support

If the previous owner's Apple ID does not appear there, you can wipe it yourself, and start over, to remove any possible presence of the previous owner's Apple ID from the device. There can be no apps that he purchases left on the device either as those will be tied to their Apple ID and cannot be used by you.