The default backup time for a WHM/cPanel server is embedded in the root CRON. To change the time of the backup, you need to consider two CRON entries:
The Legacy Backup CRON:
0 1 * * * /usr/local/cpanel/scripts/cpbackup
The “New” Backup CRON:
0 2 * * * /usr/local/cpanel/bin/backup
The legacy backup CRON runs at 1AM, and the “new” backup CRON runs at 2AM. I generally like to shift both on them in the same direction. For example, if I make the new backup CRON 4AM, I make the legacy backup CRON 3AM.