It is already possible to edit the metadata of an input history entry even after the initial file upload. This applies both to the Predefined Metadata entries and to Custom Metadata, except for the protected fileName attribute.
This functionality is useful in case you forgot to add important metadata when uploading the file to be processed, or if you want to adjust or enrich its metadata later on.
An update of the metadata may or may not trigger a reprocessing job, depending on the configuration of the respective processing pipeline. By default, the automatic reprocessing of the input file upon metadata update is disabled.
To reach the Edit Metadata dialog, go to the respective input history entry and click the 
edit icon on its row.
In addition, we have added a Data type selector for custom metadata entries.
For details, refer to Input History and Configuring a pipeline.