forgot screen time passcode

You will have to restore the iPhone to factory and then set it up as new. You will not be able to use a backup, since the backup contains the passcode. See this for instructions, 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.

Follow the instructions in this Apple support article—>If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support. Do not restore a backup, because backups will also put the screen time passcode back.

One thing you can do to minimize (but not eliminate) data loss is to go to Settings//{your name}/iCloud and turn on all of the switches to sync data to iCloud. Wait an hour or 2 for the sync to complete. After restoring turn the same switches back on.