You can update Lifecycle Controller and BIOS using OS or using iDRAC. Please refer below link for details. It it recommended to update iDRAC first followed by BIOS. It is also recommended to perform staged update If  you are trying to update from very old firmware to latest. [URL] 
On boot my T420 reports Lifecycle Controller Update Required. When I hit F10 nothing happens. I downloaded the latest Windows installer package for the iDRAC with Lifecycle Controller plus the latest BIOS - both as exe. Can these be run from the OS and in what order? One other strange and perhaps related issue is the Dell Support Assist app wil not analyze my hardware. Its tries for about 20 minutes then reports an error.
T420 Life cycle Controller update