Today I upgraded my firmware on a X3650 server. This is quite easy if you know how to do. You can be miles away if you have an IMM installed. This works on most IBM products. I have updated bladecenters in a similar way to. To do this do the following.
Start by downloading IBM BoMC
Run the setup and extract.
Now connect to your IMM and choose to remote control your server. Mount the iso file you created in the previous step.
Reboot the server via IMM command. Press F12 in the BIOS to choose boot point. Choose the IMM image as boot device.
Agree, Press next and press stop. Otherwise it will update default settings. Choose the updates you want and press next. Now wait.
If you choose to update IMM Firmware as well you may loose connection. Depending of the version you may have to use different browsers after completion. When done press finish and reboot.
You can reboot from IMM itself if you can not remote control. However the upgrade may take a long time so it is good to know what actually is on the screen.