Where do I reinstall ios

whats the situation? why do you feel like a reinstall is needed?

here are some resources for that. just keep in mind of consequences if no backup is made prior to restoring/reinstalling iphone/ios. consequences being data loss.

back up device before you do anything.

• How to back up your iPhone, iPad, and iPod touch - Apple Support

following this article answers your question

• How to factory reset your iPhone, iPad, or iPod touch - Apple Support
• Restore your iPhone, iPad, or iPod to factory settings - Apple Support