If you have setup a Backup to run in WHM -> Backup -> Backup Configuration and would like to force it to run immediately instead of waiting for the backup schedule you can do so through SSH.
1. Login to your server via SSH
2. Type: /usr/local/cpanel/bin/backup –force
3. This will report that it is starting the backup and creating a log file.
4. If it does not return with that information after running it, this means that there is most likely already a backup being taken. You can check to see if a backup is being run by typing: ps aux | grep backup
If it has been running a long time (e.g. it may be hung) you can kill it by finding the PID from the ps aux output, and typing: kill -9 PID