I have found success in the past using cprid_util (sk106490) to change the password when below R80.x. As Dameon mentions, the Script Repository is great for this now.
Below are the steps I had used in the past.
1.) /sbin/grub-md5-crypt
a.) Enter Password 2x Times
b.) Copy Hash displayed on screen
2.) $CPDIR/bin/cprid_util -server 10.0.0.x -verbose rexec -rcmd /bin/clish -s -c 'show users'
3.) $CPDIR/bin/cprid_util -server 10.0.0.x -verbose rexec -rcmd /bin/clish -s -c 'set user <USERNAME> password-hash <HASHED PASSWORD FROM STEP #1>'
4.) $CPDIR/bin/cprid_util -server 10.0.0.x -verbose rexec -rcmd /bin/clish -s -c 'save config'