Navigate to Device > Users > Settings > Authentication.
Select Enforce password complexity for One-Time Password to meet complexity requirements of the one-time password.
For the One-time password Email format, choose an email format for :
Plain Text
HTML
For the One-time Password Format, select the password format:
Characters
Characters + Numbers
Numbers
In the One-time Password Length beginning and ending fields, enter the minimum and maximum length of the password. The length must be between 4-14 characters. The default for both fields is 10 characters.