As of this writing, I have only two plugins in the WordPress.org repository. I periodically update these plugins, but not often enough to memorize all the steps required to get things right on the first try. So for my own future reference (and maybe yours as well) here are all the steps laid out in order.
1. Check out the repository
If we don’t have a local copy, this is the command we’ll need:
svn co http://plugins.svn.wordpress.org/your-plugin-name my-local-dir
The my-local-dir folder will be created if it doesn’t already exist.
If we already have a local repository, we use this instead:
svn up
2. Make changes
We’ll want to modify the files in the /trunk folder.
3. Update version number
The version number is located in two places. We’ll want to make sure to update bot of them. First, it is listed in the header comment of the main plugin file. Second, it is located in the readme file.
4. Update Changelog
Document any new changes in the readme file.
5. Commit changes
Commit changes to the WordPress.org repository with the following command:
svn ci -m "Commit message for new feature"
6. Tag latest version
WordPress allows you to archive each version of the plugin so that is will still be available for download even after it has been replaced by a newer version. To archive the current version, copy the /trunk folder into the /tags folder and give it a version number. The examples below use a version number of 2.0.
svn cp trunk tags/2.0
Finally, commit the tagged version to the WordPress.org repository with the following command:
svn ci -m "tagging version 2.0"
Credits
All this information is already available, but not all in one place. I gathered information from the following pages to compile this post: