9/13/21 security update crashed my iphone 6s

Hi, I dutifully started the install process of the 9/13/21 security update on all of our Apple devices after the urgent warning went out yesterday. My husband's iPhone 6s started the download, then turned off, then turned back on with the Apple logo on the black screen and has been like that since last night. We've tried the force restart (holding the sleep/wake and home button), but it just shuts off and comes back on with the logo. The phone was working perfectly up until this update last night. Has anyone had a similar issue? Does anyone have a solution? Couldn't get a Genius appointment today so I'm reaching out here. Thank you!