Since my IPhone started giving the feature that I should update my IOS 14.7.1 to 16 or more I can’t download anything again. I can’t access any application that requires more than that version . I am not the first user of the phone. I sent it to repairer and they said they can’t help

[Re-Titled by Moderator]

Original Title: How can I update my IOS from 14.8 to a higher one on my Iphone 8 since I am not the first user of that phone?