The Updated Project Handbook offers guidelines for the members of the EOSC-SIESTA project. It covers the most important aspects of the project, including its management structures, project reporting, and working practices. This version specifically gathers all modifications made to the initial Project Handbook since the project's inception 18 months ago, providing a comprehensive and up-to-date reference. Some aspects within this document are only touched upon, as they are or will be more formally laid out in the Grant Agreement, Consortium Agreement, and DataManagement Plan.