If the diagram is a class diagram there is no relationship to be taken from your question.
The relationship between these two classes would be best show in a Sequence Diagram or an Activity Diagram. These show method calls etc, a method may be a member of a class in a class digram but it wouldn't really be expressed as a link. Unless you specifically wantto highlight this method, you could then put a usage link to the method on the class. But its not always good practice to do this for every method as you end up with a very messy class diagram.
It's always best to show these relationships in the diagrams you have to show the usage of the method.
EDIT:
How does the class A instance relate class UpdateRequest instance?
Q - What relationship are you trying to show? A - Method/function call.
Some class calls updateResponse and the information flow is an UpdateRequest instance.
Therefore... From left to right
SomeClass --| A
| | a = new UpdateRequest |
|<-----| |
| |
| updateResponse(a) |
|------------------------------------>|
Your sequence diagram might look something like this. There is no specific relationship drawn between class A and UpdateRequest as it is simply a piece of information that flows through the method call you have in your example. If the class was a member of the class, then the relationship would be shown directly on a class diagram, not applicable here. All the classes might (should) shown on a class diagram seperately, this can then be used as a reference point about all the objects in any diagram for those that need to analyse the design.