I don’t have computer to update to my iPhone

If the iOS version is not too old you should be able to update it by going to Settings, General, Software Updates.

Check all of these carefully:-

1. you are on a good Wi-Fi, (from 15.4 iPhone 7 up, you may be offered cellular update)  and  that the router is connected to the internet.
2. Plenty of spare storage – 6GB at least.
3. on charge with more than 20%, OR 
4. off charge with more than 50%
5. low power mode turned off. 
6. Check you have no Beta profile installed. (Check Settings, General, VPN & Device Management)
7. no VPN installed (Check Settings, General, VPN & Device Management)

Resources:-

Update iphone

Update your iPhone or iPad - Apple Support