To upload a backup, open Hosting → Manage → File Manager (Beta) section:
Once there, select the Upload button in the upper right corner:
There you will be able to choose a file (archive for example) or a folder:
Make sure that your archive has one of these formats: zip, tar, tar.gz, tar.bz2, tar.xz, tar.lz4 or tar.sz
That's it! Now you know how to upload a backup with File Manager 😊
NOTES:
Upload limit for File Manager (Beta) is 100 GB