1.
Try to free up iPhone device storage see this Apple articles below:
If you need more space for an update on your iPhone, iPad, or iPod touch
If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support
How to check the storage on your iPhone, iPad and iPod touch
How to check the storage on your iPhone, iPad and iPod touch – Apple Support (UK)
Clear Other storage on your iPhone or iPad
Clear Other storage on your iPhone or iPad - Apple Support
2.
What's the difference between device storage and iCloud storage?
What's the difference between device storage and iCloud storage? - Apple Support
3.
Check iPhone type & iOS version -> Go to: Settings -> General -> About.
If your iPhone is eligible for an update, you can check by reading this Apple article:
iPhone models compatible with iOS 15.6
iPhone models compatible with iOS 15.5 – Apple Support (UK)
Then go to: Settings -> General -> Software Update
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.