How to release update your WordPress plugin
Make changes to the trunk. The trunk should only have your working copy.
Update your readme.txt file to describe the changes you have made, but DO NOT change the Stable Tag. This number is pointing to an existing directory inside your repo’s tags directory.
Save your files, then commit your changes to the SVN repo:
svn commit -m "My new version is ready"
Tag the new version (0.5 in this example) using the SVN copy command to copy the trunk new a new numbered directory in tags:
svn copy http://plugins.svn.wordpress.org/your-plugin/trunk http://plugins.svn.wordpress.org/your-plugin/tags/0.5
Remember: the tagging operation is just a copy operation. By tagging the version, the readme.txt file can be updated with the new stable version tag.
Stable tag: 0.5
That number acts as a pointer to the corresponding folder inside of the tags directory (it would point to http://plugins.svn.wordpress.org/your-plugin/tags/0.5 in this example).
Commit your changes to the readme.txt file. This will ensure that the Stable Tag attribute points to the newly created version:
svn commit -m "Updating the stable tag"
That's it... high five