if you have all your media on your iTunes and other files backed up in itunes (backs up on sync by default) then you shouldn't lose anything
factory reset it or restore it
Actually, It doesnt. 'Cause its the purpose of backing up the data. After the update, Your iphone will restore then it will put back your old data.
the same way with all of the other iphones
iTunes cannot help you do this. But FonePaw iOS Transfer allows you to sync/transfer/copy files from iPhone to iPhone, iPhone to iPad, iPad to iPad, iPhone to iPod etc.If you want to transfer data or files from one iPhone to another one, you can use tools like AnyTrans to make it. It won't erase the previous data on your devices. With it, you can also transfer purchased and non-purchased items from iPhone to computer, to iTunes.
All you have todo is attach the iPhone to your computer insure iTunes is open and then select restore to factory settings options, which will reset the handset and erase ALL previous data.
No. Replacing the battery will not wipe the phone. Make sure the phone is off before you disconnect the battery though!
If you want to format it with out using iTunes then go to setting on your iPhone then to general then to reset then select erase all content and settings it should ask you to enter your pass code then it will make sure with you that you really want to format your iPhone to its original state and after a few minutes your done. Here is a way about how to format iPhone. Settings--->General--->Reset--->Erase All Content and Settings and wait for the iPhone to format itself. Before format your iPhone, you'd better backup your data.
Yes there are many Apps out there ex: Find My iPhone and also if you have iCloud account setup you can remotely erase the data from lost/stolen iPhones
I can offer some help for unlocking your iPhone, The first method is unlocking iPhone without Passcode via iTunes. At first, connect your iPhone to the computer and open iTunes on Windows. Then choose your locked iPhone and click Summary on the left side of the menu screen and select the restore iPhone button. After that, confirm to Restore iPhone to finish unlocking your iPhone. The second method is removing iPhone Lock without Passcode or Face ID via iCould. Firstly, Go to iCloud through a web page and enter the same Apple ID that have logged in to your locked iPhone. Then choose the Find My feature on the page. You will see all the devices in a list, including your iPhone that can't be unlocked through passcode. At last, Select your iPhone and click Erase iPhone to remove the lock. Again click Continue to go through the process. Write down your phone number when required. Wait till the step is completed. The third method is to use unlocking tools such as MagFone iPhone Unlocker, You need to run MagFone and connect Your iPhone to the Program first, Then put your iPhone into DFU Mode or Recovery Mode and download and verify Firmware Package, After downloading, choose to unlock your iPhone without passcode. Sincerely wish these method will be helpful to you.
you should probably restore your iPhone. Plug it in go in iTunes, click on your iPhone and press the restore button. THIS WIL ERASE EVERYTHING ON YOUR iPhone IF YOU HAVE INFORMATION ON IT THAT YOU NEED RIGHT CLICK YOUR IPHONE AND PRESS BACKUP BEFORE RESTORING. The backup may contain whatever is breaking your app installs so if you still are having trouble bring it to an apple store and have them backup just what you need. Most likely this wont be necessary and just backing up and restoring should work.
It shouldn't because all your doing is turning it on and off
You cannot bypass the "Firmware" password, you HAVE to erase and reinstall the OS if you forgot the "Firmware" password.