How can I update iOS 15 to my phone iPhone 7 Plus ?

1.

How can I update iOS 15 to my phone iPhone 7 Plus ?

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

You need about 7 GB device storage and good Service & Wi-Fi.

If they are On, turn Off Hotspot & VPN.

2.

I want to transfer money from my Apple Pay to my account and it won’t let me

See -> Transfer money in Apple Cash to your bank account or debit card

See also -> If you can't send or receive money with Apple Cash

Mommymommy6 Said:

"How can I update iOS 15 to my phone iPhone 7 Plus ?: I want to transfer money from my Apple Pay to my account and it won’t let me"

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Transferring Apple ID Account to Bank Account:

You cannot do that as of this reply.