Not able to install 15.3 os in iphone 6s
Unable to install update 15.3 os in iphone 6s
iPhone 6s, iOS 13
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Optimize the range of the mobile network, see this Apple article: If you see No Service or Searching on your iPhone or iPad
https://support.apple.com/en-us/HT201415
Check also the Wi-Fi network, see the following Apple articles:
Connect to Wi-Fi on your iPhone, iPad, or iPod touch
https://support.apple.com/en-us/HT204051
If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network
https://support.apple.com/en-us/HT204051
Low Power Mode is On.
Charge the iPhone, Wait until the battery is fully charged (100%).
The Low Power Mode will turned off automatically.
Then you can perform the update.
You need about 6GB device storage, see this Apple articles below
If you need more space for an update on your iPhone, iPad, or iPod touch
https://support.apple.com/en-us/HT203097
How to check the storage on your iPhone, iPad and iPod touch
https://support.apple.com/en-gb/HT201656
Clear Other storage on your iPhone or iPad
https://support.apple.com/en-us/HT211686
What's the difference between device storage and iCloud storage?