I�m not really understanding your relationships, and basically you have a set of records and need to update a field called something like Related Parent and you need to mass update that field to a new value.
It seems to me that you either already have a report link field or can make a report link field to the find that set of records. And then you can use an Action to do the mass update.
Pre-action can be triggered by some kind of button or checkbox field. The rockers you were sitting on would need to know the record ID for the new value.
So in the spirit of low no code I think that is a no code solution
