First you have to download the change admin script to your /etc/phpshellsessions with the following command:
# fetch -o /etc/phpshellsessions/ https://raw.githubusercontent.com/pfsense/pfsense/c07e853bb4a67a3b728b7546b36801eaef770c19/etc/phpshellsessions/changepassword
And then you run the the following:
# pfSsh.php playback changepassword
It will ask you the new password and to confirm the new password for the user. After you complete the above you can try log in the webgui with the new password.
Have a nice day.