Since the Unique code is based on the PLAN code added. 

You can either update the plan code by editing the plans details 

Or

To individually update the Unique code for each customer you can:

Go to CRM 

Find the Customer > Click on Digigold icon > Click on Plan

On the Top Right Corner > Update the Unique Code of the Customer. 

Please note Unique Code can be updated based on the each plan and each customer manually. 

If you want to automate assigning unique code then you can set up the Plan Code.