In literature, the exposition is defined as a passage where we are given the background to the story that is about to unfold. Given that definition, you can probably tell that the exposition in this scene comes in the conversation between Horatio and Marcellus.
In this conversation, they go over what is going on in Denmark during this time. They discuss how the late King Hamlet had defeated the Norwegians and how now the new king of Norway was going to attack Denmark. This is the background to the story that is still to come.
In a sense, you can also say this is exposition in that it sort of tells us (though not explicitly) that something is wrong in Denmark. This sets us up for the story of Gertrude and Claudius and Hamlet's dislike of their marriage.