Question about defining actions of operations on attributes

I’m new to UML and have been spending a lot of time over the last few days learning it… I feel I have a good understanding of how most of the diagrams function, however this one thin is still eluding me… I have created a class diagram, so, for example, I have a class with a few associations and one operation. Lets say that the operation is to “create a new value for the association”. I can’t figure out, then, which diagram would be used to specify the relationship between this “create new value” operation and a specific association, from what I can tell it would not be done in the class diagram.

Thanks for any advice on this.

It sounds to me you want a state machine diagram, which can depict the values of attributes, that is, the state of the objects illustrated.