Identify the phone
Confirm the iPhone model and model number before comparing eligibility. Similar-looking generations can have different status.
Check the installed version
Open Settings, General, then About. Use Software Update to see whether Apple offers a newer release for that phone.
Read the release record
Apple lists security releases and affected products. The compatible-device list controls whether a release applies to the exact phone.
Primary sources
Important
Device support and safety information can change. Recheck the linked manufacturer and regulator evidence before making a purchase or safety decision.