Hello Panida,
Looking at Mike's post, I'm guessing he went to Settings > Site administration > Users > Permissions > Define roles. I looked in two places under Define Roles: student and authenticated user. There is a setting in both places called "change own password."
I'm afraid clicking the setting to "Allow" didn't work for me. I'm running 2.71 and having another issue which may be related and complicating things: I'm unable to send emails to students. Trying to fix the second issue, I've played with the SMTP settings and contacted my hosting provider: Bluehost, but i haven't had much luck. I haven't contacted the techs about the forgotten password problem. I want to make sure it is a MOODLE issue and not related to Cron or something else on the server before I trouble them. On that note, I'm curious if Mike's emails were going out before the Forgotten Password problem and if they are working now. This might help me figure out things.
If anyone has any ideas, I'd be very grateful. I have had a line of students out my door everyday this week asking for help with their forgotten passwords. MOODLE's password requirements of eight characters with an upper and lower case and special character seems to be a challenge for many of the students.
Best wishes,
John