Problem with objects in OOP

Hello dear Community,

I am kinda stuck with this problem for a long time time, and I really need your help. I am new to OOP so be gentle please :)

I have defined several instances of classes, and I need them all to have one property in common, that will be defined through a method of one instance that I will find thanks to a certain value of a property it has (actually, every instance is from an object, that share the same upper class, so they share some properties, but have different ones as well). Is there a command that will allow me to search upon all objects, and find the one with that special property and finally define the common property for all instances? Maybe there is a easier way to do that, but I can't figure it out!

Thank you a lot!