How can I remove the restrictions if I forgot the passcode for them?

Hi there,

If you've forgotten your screen time passcode please follow these instructions

1. Go to Settings > Screen Time. 
2. Tap Change Screen Time Passcode, then tap Change Screen Time Passcode again.
3. Tap Forgot Passcode?
4. Enter the Apple ID and password that you used to set up the Screen Time passcode. Forgot your Apple ID password?
5. Enter a new Screen Time passcode, then enter it again to confirm.

See this link for further reference

If you forgot your Screen Time passcode - Apple Support

When you set a passcode, you are prompted to provide an Apple ID/Password in case you forget your Screen Time Passcode. If you skip this step, you are once again prompted with a warning that the Apple ID/Password will be required to reset the passcode in case you forget it. By ignoring these 2 warnings, you have taken responsibility for remembering the passcode yourself without any other assistance.

You can try to contact Apple directly, but it will not be their responsibility to determine if the person who is calling had the passcode set by a parent or organizer of a family plan and they are not supposed to know the passcode. You may just need to reset the device to factory settings which will wipe all information and you will not be able to restore from a backup as that will only put the passcode back on the device. You can attempt to save some of your data by storing in iCloud before you reset the device.