If you can SSH into the My Cloud you might be able to update the firmware using SSH. Here is one thread that discusses how to do so.
The real issue though is to determine why you cannot access the Dashboard in the first place. To that end is this My Cloud one that was unbricked previously? If so it is possible upgrading from v3.x firmware to v4.x firmware caused the Dashboard to be inaccessible. This issue is discussed in the following thread.
Other things to look at to troubleshoot when the Dashboard isn’t reachable.
Can the My Cloud be reached by Windows File Explorer or Mac Finder?
Do any of the WD software or apps access the My Cloud?
Is the My Cloud connected to the local network?
Is the light on the front of the My Cloud blue?
Are both LED’s on the back network port of the My Cloud lit or flash?
You may want to watch this video and see if it helps.