You cannot upload label files yourself. This has to be done by us. Once we have uploaded the files you will receive an email request to approve the files from within your account. The labels will not be released to any orders you place until them have been approved.
You can send your files direct to our design team at firstname.lastname@example.org
If your files are going to be large then we would suggest either a shared online folder transfer (e.g. dropbox) or using a file transfer service such as We Transfer - https://wetransfer.com