Reset iPhone

Follow the steps provided here:--> What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

When done, that device lets you start from scratch.

Regards

Giulio

Using the procedure I suggested ALL data, media, documents, apps are deleted from the iPhone.

You need to perform a full backup before starting (as written in that support article).

Regards

Giulio