Before changing an existing user, make sure you have read and understand the considerations noted in Naming Convention for Users.

Considerations:

• Windows will not rename the home directory, but security permissions will be updated to the new username
• Any applications that may have depended on the old username may not be updated by the User Account wizard.  Be sure to test any applications before making production changes
• Renaming a User Account Does Not Automatically Change the Profile Path

Log into the system using an administrator account that is not the account being renamed. From a command line or PowerShell, run the User Accounts Management wizard.
 

PS C:\Users\Administrator> netplwiz

Select the user, then Properties

Change the User name value to match the Username in the JumpCloud directory.  Click OK and restart the system.