Hello,
The available symbols of an object depend on its object type. The symbol "Cluster" belongs to the object type "Cluster/data model". The symbol "Document" belongs to the type "Information carrier". Since the type of an object is immutable over its life-cycle you cannot change the symbol of the cluster to the document symbol. You cannot make an apple to be a pear by painting it yellow.
The cluster/data model represents a data structure or business object, while an information carrier represents a type of media. So in this case you have to create a new document and drop the cluster. If you systematically want to replace all objects of one type by a different type, there is a report that can assist you. It also helps you to replace the connections the objects have with new connections.
If you are not the method owner in your organization, consult with the method owner on this effort first. The use of such a report has a major impact on the structures in your database. Reports or queries looking for the clusters won't produce the expected output any more.
BTW: It looks like you are working with the "Entire method" filter. It is good practice to have some tight conventions which symbols and connections to use in which model type so such mistakes like choosing the "wrong" symbol are less likely to happen. A custom method filter for your organization will enforce these conventions.