How do I reset Screen Time password on iPhone?

If you forgot your Screen Time passcode - Apple Support

Follow the instructions in the link provided.

If you are unable to reset your Screen Time Passcode with your Apple ID then you may have skipped the option while setting up Screen time Passcode. See the pic below

If you have forgotten your Screen Time passcode. There are two options.

If you are unable to reset the Screen Time passcode from this-> link, then you will need a computer for option 2. If you don't have one, borrow one from a friend.

Option 1

Use this link to reset your passcode --> If you forgot your Screen Time passcode - Apple Support

Option 2

Forgot? Click below to reset…

Click->If you forgot the passcode for your iPhone, iPad, or iPod touch, or your ...

If you have chosen option 2, then regarding your data, if…

1. You have an iCloud or iTunes backup, I am sorry, there is no use of a backup because if you restore from your backup, your Screen Time Passcode will be back. 
2. You have synced your data to iCloud, then what you will get back is what you have synced to iCloud and uploaded to iCloud Drive; the information that is synced and stored in iCloud are, Contacts, Calendars, Bookmarks, Mail, Notes, Voice Memos, shared photos, iCloud Photos*, Health data, call history, and files you store in iCloud Drive. (*-If you had turned on iCloud Photos, your content is automatically stored in iCloud.)
3. You have imported your Photos & Videos to Mac/PC, then they are safely residing there
4. You have uploaded some of it to Cloud Drives using the Files app (iCloud Drive, Google Drive, OneDrive, Dropbox, Flickr, box, AmazonDrive, Adobe Creative Cloud, etc)
5. If you have not imported, nor have you synced or uploaded your data to cloud drives before forgetting the Screen Time passcode, then I am sorry, all your data is lost.