The username and password are different for each video you upload, are different from your site username and password, and are only valid until you confirm the upload (I think).
You'll need to request upload permission on each video you want to upload to get the username and password to use for FTP.
If you are trying to replace a LOCAL upload with another file, you'll need to request a video deletion
to get the LOCAL link removed. You can request that just the LOCAL link be removed (instead of the entire catalog entry). After that, you'll be able to request upload permission for that video again, and then you can upload the new version.