forgot password on iPhone

Passcode locked?

(< iOS 15.2) See this Apple article: If you forgot your iPhone passcode

If you forgot your iPhone passcode - Apple Support

(iOS 15.2 and newer) 

How to remove your iPhone, iPad, or iPod touch passcode without a computer

How to remove your iPhone, iPad, or iPod touch passcode without a computer - Apple Support

Forgot Password?

See -> If you can't connect or sign in to iCloud

If you can't connect or sign in to iCloud - Apple Support

See this Apple article: If you forgot your Apple ID password

If you forgot your Apple ID password - Apple Support

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support