When you look at the plugin description (and other sections), it all comes from the current version README.TXT file.
So if you upload a changed version of README.TXT to the current version’s repository (not changing the repository version number), the newly updated README.TXT file information is shown on the page.
This is great for updating the ‘tested to’ version numbers, when you didn’t make any code changes to the plugin.
More info here (and other links from there).