Workflow diagram is contained in workflow.mmd. Its definition is written in Mermaid.
In order to regenerate the workflow diagram from source, use the commands:
# Install Mermaid CLI
npm install @mermaid-js/mermaid-cli
# Generate the diagram
./node_modules/.bin/mmdc -i workflow.mmd -o workflow.png
If you want to adjust the diagram and see the result applied live, use Mermaid live editor.