Metadata can be edited for a document that under a workflow process. In order to use this feature, the option "Enable editing document metadata when workflow is active" needs to be enabled. You need to enable this setting in the System Administration > Permission Settings area.
Any user that has access to the document undergoing a workflow can edit the metadata. If you do not want users outside of the workflow editing metadata on the document, select the Hide all versions of the document check box in the workflow template until the document has been approved in the workflow process.
To edit the metadata of a document undergoing a workflow
Select the document undergoing a workflow in the Library.
Select Metadata & File Properties and click Edit. Make and save the changes.