Step 1:

Download DRM Utility using the following link;

https://www.dell.com/support/kbdoc/en-pk/000141551/how-to-create-a-bootable-usb-device-with-rufus-to-update-dell-servers

Step 2:

Install DRM Utility and Launch it.

Follow the Wizard to Start;

Type in search bar or select Server according to your requirement
Navigate to Repository Manager to check Job Status
Finally you will have a bootable ISO Image

Now you can create a bootable USB using Rufus or Unetbootin tool.

Boot Server from USB and process for firmware upgradation will be started automatically.