If for installed plugins the update link changed (like more or less recently when a lot of plugin developers switched to git) the update button int he plugin manager does not work. Copying the new URL into the respective form field and using the download button does //not// help, since the installation is skipped with the message: "Plugin package (0 plugins: ) successfully installed." The reason for skipping is the $overwrite=false setting in ap_download.class.php, preventing the installation of new plugins to overwrite existing plugins/templates.
This is at odds with the documentation which states for exactly this situation:
"...Note: This only works when the plugin maintainer always uses the same URL for the plugin or provides some redirection to the latest version through a common URL. //If this is not the case just get the new URL and install it like a new plugin. The Plugin Manager then will update your old plugin.//" (emphasis added)
Attached is a small patch with an additional notification of the user.