Can I update my iPhone 7 above iOS 15.8.3?

Hi, I have an iPhone 7. It says the iOS is up to date at 15.8.3.

I’ve been reading about going into VPN configuration to check for Beta settings, and you may be able to update iOS.

It says my VPN is not connected. Under ‘Type’ it says IKEv2.

is there a way I can go into VPN settings (or elsewhere) and enable the phone to accept updates higher than 15.8.3, or is that as high as it can go? Is there any form of external hardware you can use to update the iOS?

Besides the cloud, is there a way to add storage to a 68GB phone that is full?

Thanks! Patti

[Re-Titled by Moderator]