If you have OMSA installed on the server, you can run the below command to set the iDRAC Root user password.

racadm set idrac.users.2.password calvin

Above command will work only if your iDRAC FW is 1.30.30 and above. You can run below command if above command is not supported by the firmware

racadm config -g cfgUserAdmin -i 2 -o cfgUserAdminPassword calvin

You can also set the root password from iDRAC Setting Utility during boot. For this you can reboot the server and go to iDRAC Settings by pressing F2 during reboot. In iDRAC Settings page you will get an option to reset root user password