update i phone 7

Try to optimize the range of the mobile network, see this Apple article:

If you see No Service or Searching on your iPhone or iPad

If you see No Service or Searching on your iPhone or iPad - Apple Support

If you still don't have a 'service' after that, see -> iPhone 7 Repair Program for "No Service" Issues

iPhone 7 Repair Program for "No Service" Issues - Apple Support

i have also tried to use my hotspot

For update, if they are On, turn Off Hotspot & VPN.

it gives error code 1667.

See this Apple article: iOS update and restore errors

iOS update and restore errors – Apple Support (UK)

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Check your hardware

If you see error 53, use these steps.

If you see one of the errors below, you may have a hardware issue:

• 1, 3, 11, 12, 13, 16
• 20, 21, 23, 26, 27, 28, 29, 34, 35, 36, 37, 40, 56
• 1002, 1004, 1011, 1012, 1014, 1667, 1669

Here's what to check. Try again after each step:

1. If iTunes is already open, quit and reopen it.
2. Check your third-party security software.
3. Make sure your USB cable, computer and network are reliable. Then try twice more to restore your device.
4. Check for outdated or modified software. 

If you still see the error, contact Apple Support.

Then try again to update to ioS 15.6

See the following Apple articles:

Update your iPhone, iPad, or iPod touch

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

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 5 GB device storage and good Wi-Fi.