Below are two different guides on how to update FFU files to contain a newer build, should also allow for modification to the FFU Contents (further details on this after I test to confirm the possibility for modifications)
Note: I have only tested this on a Lumia 950 (RM 1104) and Lumia 930 (RM 1045). There may be issues so open an Issue if any are found
This repo was made private while I figure out an issue with W10M FFUs, this has now been resolved and it's now public again!!