If your requirement is to modify non-root user passwords, this document will guide you through the process.

Mist UI only provides a menu option to change the root account password as of today. Any additional local users must be updated through Additional CLI commands.

Configuration example:

1) Imagine you have an user named Rick and you want to change its password.

root@sw> show configuration | match Rick | display set
set system login user rick uid 2200
set system login user rick class super-user
set system login user rick authentication encrypted-password "$6$KeOgnxmf$L46SPlPeij/yXALSKL/rRXRi5.U6Zorss5Emue4sU/mhC7/rPzGM4LVbdsaCyWW7hKVTu4USe3jJVSXEPhAbg0"

2) First login to a switch and change the password from the CLI (you can use any switch running same OS for this step):

{master:0}[edit]
root@sw# set system login user rick authentication plain-text-password
New password: 
Retype new password: 

3) You will notice the previous encrypted hash has changed:

{master:0}[edit]
root@sw# show | display set | match rick
set system login user rick uid 2200
set system login user rick class super-user
set system login user rick authentication encrypted-password "$6$Ngr7anyl$D8y4PfipsQkRBpzTyHA9SsTKKPzmdOqdKhVChuFjWG7LBdnrcrgTUTLbnINklKWvkpOUN.w/7hZF0u4rkhes/."

4) Now grab these new commands with the modified encrypted password and paste them in Mist Additional CLI commands and Save it:

Note:
Discard the commands inserted into the switch CLI, we simply needed to get them into Mist:

{master:0}[edit]
root@sw# rollback 0
load complete

{master:0}[edit]
root@sw# exit
Exiting configuration mode