Update iPhone to the latest iOS version
Open Settings
Tap General
Tap Software Update
Install any available update
Delete suspicious apps
Press and hold the app icon
Tap Remove App
Tap Delete App
Clear Safari website data
Open Settings
Tap Safari
Tap Clear History and Website Data
Check for unknown configuration profiles
Open Settings
Tap General
Tap VPN & Device Management
Remove anything unfamiliar
Restart iPhone
Open App Store
Review installed apps
Reinstall only trusted apps
Change important passwords
Enable two-factor authentication
Contact Apple Support if problems continue