The minor can send them a letter asking to withdraw, and the I-130 petition can send a letter requesting to withdraw the petition.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.
If the Petitioner withdraws the I-130 Petition, then the I-485 will be denied. If you are the Petitioner, you can only withdraw the I-130; you cannot "cancel" the I-485 because that was filed by the Beneficiary.
The information provided is given for general purposes only. It is not meant to provide any specific guidance or give any legal advice. It is recommended that you bring your specific situation to a licensed attorney for a more complete and thorough evaluation.
The petitioner can withdraw the I-130 and then the guardian for the minor can withdraw the application for adjustment. It's a better idea to withdraw the application than to allow it to be denied.