How do I reset my screen time password if I can't remember it?

See the following from Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

If you can't remember a restrictions passcode that you created on a previous version of iOS, or you forgot your Screen Time passcode, you need to erase your device, then set it up as new to remove the passcode. Restoring your device won't remove your old passcode.