The first place to start is by reading the documentation: Armbian Developer Guide. The developer guide walks you through the armbian build process. The entire armbian build process may be more than you are looking for (not knowing what changes you are looking to make), but with an overall understanding of the armbian build process (simply being able to rebuild the image you are already running) you will likely be able to figure out a path forward for your needs, or at least be able to come back to the forums with a more specific question based on greater understanding of the system.
