To see which PBIs belong to which release, you need to first link the PBI to the respective product. Once linked, only the releases that are planned for the product will appear in the release dropdown.
If an epic or user story does not belong to a particular product, you still have the option to include it in one of the future versions. When you click the Release drop-down list, all products and all releases will be displayed so that you can select a release version that this PBI belongs to.
To view and edit the release click on the title or the details symbol on the right side (Quick View & Detail View).