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
See this Apple article: Upgrade to iCloud+ -< Upgrade to iCloud+ - Apple Support
See also: iCloud+ plans and pricing -> iCloud+ plans and pricing - Apple Support
Manage your iCloud storage
Manage your iCloud storage - Apple Support
3.
Check iPhone model & iOS version -> Go to: Settings -> General -> About.
If it is an iPhone 6 (and other older iPhone type) is an outdated iPhone it can’t be update to a higher iOS (13 and later).
The last security updates iOS 12.5.7 -> available for iPhone 6.
If it is iPhone 6S Plus you can update to iOS 15.7
Go to Settings -> General -> Software Update -> Download and Install.
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