HOW to format iphone

If you want to keep your data, apps, settings, etc., make a full backup of your iPhone.

Then restore your device to factory settings (this will install a fresh copy of iOS), and, at the end, set it up as new:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

When done, set your iphone as if it were brand new.

If you want to restore your data, do it restoring from your previously made backup.

Regards

Giulio

EDIT:

After the first line in my previous post, please read:

"Restore your device to factory settings (this will install a fresh copy of IOS), and, at the end, set it up as new:"

Then Follow the support guide I provided.

Giulio