UC926/UC926E/UC924/UC924E/UC923/UC903/UC862/UC842/UC860/UC840 support "EHS upgrade" feature. Please upgrade EHS as followed steps.
Step 1 : Login webpage: Setting -> Preference -> Zero Touch Type. You can see two items: "EHS" and "Normal". Choose "EHS", Save .
Step 2 : Set a DSS key to Zero Touch , Save.
Step 3 : Back to idle page on LCD and press "DND" button to set the phone to DND mode to forbid the incoming call in the upgrade process.
Step 4 : Press the DSS key which you have set it to "Zero Touch", then you can start upgrade by LCD. And you will see the page as followed on LCD.

Page one:

You have 5 seconds to press the "OK" button. If you press "Cancel" button or do nothing in 5 seconds, the Updating will be canceled.

Page two:

Please connect EHS then press "Start" button.

Page three :

You can see the information " EHS Version:V1.2" at the top of this page. It's the current version of EHS. When you finish upgrading, you can check whether the EHS have upgraded successfully. According to the prompt, you should reconnect EHS, then the process of upgrading needs several seconds.
Please Note: In this several seconds (upgrading process), the phone does not want any action to disturb its process.
So, please don't do anything with the phone in this upgrading process.
Several seconds later, you will see the page as followed :

Page four:

It means that, updating has done, then you can check the version on EHS at page three or webpage of the phone .
If the EHS has not upgrade successful, please try to upgrade again.

Page five:

If you have another EHS to upgrade please connect EHS and then press "Start" button. And if you want to finish upgrading, please press "Cancel" button .

Page six:

If there is something wrong that causes you to see this page, please reconnect EHS and press "Start" button.