This document provides a tutorial on using Mercurial (hg) to manage revisions and fix mistakes in a source code repository. It demonstrates how to revert changes to specific files, clone a repository for experimentation, commit and push changes, view commit logs and file statuses, compare revisions, backout commits to undo changes, and merge the backout result. The key steps shown are reverting files to previous versions, cloning a repository branch to experiment freely, backing out commits to undo changes from the past, and merging the backout result.