Last few weeks I tried to find out answer for this question, but I didn’t. I decided that server has this option only for Windows. Now I stumbled upon a solution.
Enable download client buttons in web interface for Linux server
For enable buttons “Download client for Windows” and “Download client for Linux” in web interface at Linux UrBackup server go to https://hndl.urbackup.org/Client/ and download all files in the update folder of the current client to /var/urbackup on UrBackup server.
The easiest way to do it (run a command at the server):
wget -r --no-parent --level=2 --reject="index*" --no-directories https://hndl.urbackup.org/Client/latest/update/ -P /var/urbackup
It downloads latest version of client. Change command if you need a different client version.
I tested it at UrBackup server 2.4.12, debian 10.3 64bit / Linux 4.19.0-8-amd64).
More information at the documentation: 10.1 Manually update UrBackup clients