There are a couple of threads in this forum that have more information. If I knew how to link them I would. Search under "iec 62304" and "software validation" . The link inserted is an article from Quality Digest.
References: IEC 62304 - Medical Device Software, Software Development Lifecycle Procesess (SDLC)
Principles of Sofware Validation - Guidance for Industry.
http://www.qualitydigest.com/print/7640
Suggested Steps:
1. Identify the risk that the software has on the performance of the device. (Iso 14971 and IEC 62304).
2. The release of the update/fix is in the Maintenance phase of an SDLC, but it still needs to follow the product realization processes as someone suggested above. (21 CFR Part 820/iso 13485) A new version# or some other sort of identification # should be assigned.
3. For the field updates, keep track of the serial# of the device and the new version/or identifyer #. (21 CFR Part 820.160)
4. The user should document this change and retest the device in their environment before using, and documentating per their own change control and/or validation requirements.