For partial deployment projects where Package Name is selected, you can view and manage the list of files in the package for the release using the Package Details link.
This will display Package Details screen, adjust accordingly and click Save.
The list of files can be managed here (adding, removing, and sequencing) or in Project screen (this was changed in FlexDeploy 5.0 with introduction of Package screen at project level). New builds referencing the release will have these files in the package by default. However, the typical model is for developers to reference the release during build, and add/delete/sequence files to the package. The package details will then be updated as per what is included in build request for files. Package details is typically considered a more technical activity managed by the development team. Release Managers who have update access to the Release screens do not typically want to get involved with file-level detail.