How to reset screen time passcode?

If you mean you just want to change it, follow the first few steps outlined in the next link. If you need to reset a forgotten passcode, follow them all or use the ones for earlier devices following that.

If you or a member of your Family Sharing group have a mobile device running iOS or iPad OS 13.4 or later, or a Mac running Catalina 10.15.4 or later, read this support article for instructions on what to do "If you forgot your Screen Time passcode". Click here --> https://support.apple.com/HT211021

If you still have issues resetting your Screen Time passcode, contact Apple Support.

https://support.apple.com/contact

OR

If you are running earlier systems you will have to read this Apple Support document about what to do "If you forgot the passcode on your iPhone, or your iPhone is disabled". Click here then select your device type if necessary --> http://support.apple.com/HT204306 - Also has links for what to do for an iPad or an iPod touch.

This will erase the device. You can restore your device from a backup but the backup has to date from before you set the forgotten passcode or it too will be restored.

On the remote contingency you are asked for your Apple ID and password AND you have forgotten those:

If you forgot your Apple ID, click here --> https://support.apple.com/HT201354

If you forgot your Apple ID password, click here --> https://support.apple.com/HT201487

To jump directly to the site to help you recover forgotten account information, click here -->- http://iforgot.apple.com/

What to do after [or before] you change your Apple ID email address or password - https://support.apple.com/HT204071

If you still need help:

- Contact Apple for help with Apple ID account security "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues." - https://support.apple.com/HT204169