Textbook updates and revisions generally fall into two categories: errata and editions.
An erratum is a writing or printing error. When errors are corrected in future versions, a list of corrected errata is sometimes included in the textbook.
An edition reflects significant changes to the textbook, often to reflect developments in the discipline or a new approach to the content. Whether or not changes warrant a new edition is often subjective.
Regardless, in order for a textbook to remain relevant, it’s good practice to set aside time to review the textbook and consider making updates and revisions. This ensures that the content you’re offering reflects current developments in the field. This can be done on an annual schedule, and is often most convenient in the summer.
Authors who publish their own open textbook have direct access to the content. That means authors can make changes whenever they see fit, including to correct errors as they are discovered (the work of which can be efficiently crowd sourced by users). Libraries funding open textbook projects and supporting their publication can incorporate annual updates into the grant process and timeline development.
When a new edition is released, be sure to update all of the locations where the textbook is shared. For example, if the textbook is available through the Open Textbook Library or an institutional repository, you will want to inform them of the new edition. A simple email with a link to the latest files is usually sufficient and appreciated.
For more, see Self-Publishing Guide, Maintain the Book [New Tab].