I can’t log in to my apple account.

I can’t log in into mine apple account because i need to update my iPhone to iOS 16

No, that is not the reason.

You are sign in here .  .  .  with your Apple ID & Password.

Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.

You can’t merge multiple Apple ID accounts.

What happens when you use the same Apple ID & Password that you use to sign in here?

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See -> If you can't connect or sign in to iCloud

If you can't connect or sign in to iCloud - Apple Support

Have you forgotten your Apple ID?, Check your Apple ID via your email box, see this Apple article: If you forgot your Apple ID

If you forgot your Apple ID email address or phone number - Apple Support                  

OR see -> Having trouble signing in?

https://iforgot.apple.com/password/verify/appleid

Have you forgotten your password?, see -> Change your Apple ID password -> Change your Apple ID password - Apple Support

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

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iOS 15.7.3 is last security updates for your iPhone, can’t be update to a higher iOS.

For sure check your iPhone model & iOS version -> Go to: Settings -> General -> About.

If your iPhone is an iPhone 7 Plus you can update the iPhone to iOS 15.7.3, see -> iPhone models compatible with iOS 15.5 -> iPhone models compatible with iOS 15.5 – Apple Support (UK)

You need about 6 GB device storage and good Service & Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

See the following Apple articles:

Update your iPhone, iPad, or iPod touch

Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If your iPhone, iPad or iPod touch won’t update

If your iPhone, iPad, or iPod touch won't update - Apple Support

You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC

Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)

If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support