l forgot the screen time passcode of my iphone 5s?

You will have to restore the phone to factory, and you will have to set it up as new. You cannot use a backup since it contains the passcode. See this support document, Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

You will need to reset your your phone to clear the screentime passcode: If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support and set up the phone as New, because the passcode is saved in the backup.

Now that I've told you the official answer, a web search may turn up a less drastic way that I shouldn't post here.