How to reset an iPhone safely Apologies for the noob question but I'm looking around and finding different answers (including data not getting restored). So I would like to make sure I'm following the right process.

My girlfriend's iPhone is acting slow and freezing from time to time. 
Therefore, I would like to restore it to its initial state and start all over. 

More info: 


Her iCloud is full and not backing up anymore
She forgot her Apple ID password 
Contacts, SMS, photos shall be restored after the reset
WhatsApp data also should be restored (I think this should be done from within the app)


Ideally, I would like to: 


Back up all data through iTunes on PC
Perform a factory reset
Create a new Apple ID 
Restore photos, contacts, SMS
Fresh install WhatsApp and restore corresponding data 


Would this work as expected?
Your first step should be to head to https://iforgot.apple.com and change your Apple ID password. Since one of the methods of retrieval will be to have a link emailed to you, do this before wiping your iPhone. If you have Activation Lock enabled, you will need to provide this password anyway after wiping.

Don’t create a new Apple ID unless you specifically want a new one.

When you backup to iTunes, make sure your backup is encrypted:


This will allow your account passwords, Health, and HomeKit data to be backed up, and will encrypt your local backup with a password. Do not lose this password!

iTunes should immediately start backing up your iPhone. After that you should be safe to Erase All Contents and Settings on your iPhone, then restore your local backup via iTunes.

Do note that it is not possible (or very difficult) to selectively restore apps, settings, etc. A restore restores all your apps and settings as they were at the time of the backup. However if there was any system corruption, etc. a wipe &amp; restore should take care of such problems.