Important:  before updating ...

1. you must  include in your extension all the locales you already have in the WTS (all can be downloaded as a single tar-gz archive file). Otherwise online translated work will be lost in the upload process!

2. you must also insert the relative locale line for each language in the chrome.manifest file of your extension (formerly in the install.rdf file for previous FF versions) 

3. When creating your new xpi for the update, only the en-US locale needs to be updated with new/modified strings. You should leave the other locales as they are, please do not fill them with new strings in English. The reason is: the WTS system parses the en-US locale files and automatically adds blank areas to fill to every existing locale, and the display of these fields with yellow background makes them easier to spot for translators.

Note on TAR.GZ files: when all locales are downloaded in a single tar.gz file, the missing strings will be replaced by their English (en-US) counterparts.

Then ... 

You just have to click on Submit/update extension in the Main Menu-> Web Translation System on the left side.

In the new page that will open, you have only to select your extension and then click on "Send File".
The extension is identified by its GUID, therefore if the GUID already exists it's an update !

Note: in order to use WTS the extension must contain at least the en-US locale
In case you need en-US version of your locale, send a message on BabelZilla board (Help section) in order to get a translator.
The previous version will be replaced by the updated one, but the locales and the known translators will be archived and available on request to the Admins.
You have to be logged in order to access to the Web Translation System