If you are a system admin, click the Menu icon, select System Settings, and scroll to the bottom to select Users. Open up the user record whose password needs resetting. Scroll to the bottom, set "Require Password Change" to YES, enter a temporary password in the "Password" field, and provide the user with the temporary password. Upon logging in, the user should receive a popup instructing them to enter the temporary password you have provided along with a new password.
Are you are already logged in and looking to change your existing password? See the Changing Your Password section in the Accessing Your Flex System article. Please note you must have the proper permissions granted to follow the provided steps.