To reset the PIN, wait until the PIN screen prompts you to log into TouchDown. Press the "reset PIN" button. If you don't see the "reset PIN" button, press the device menu button and "recover PIN."
You will receive a confirmation that your request has been sent to your server. Once you get this confirmation, please log into OWA (Outlook Web Access) and go to Options, then select Mobile Devices. Select your device from the list, then select "display recovery password." This should provide all of the information you need to be connected again.
With the more current versions of Touchdown (starting with version 8.4.00058) it is no longer necessary to send the recovery password to your OWA and retrieve it there. Now you can select Menu>Recover PIN at the PIN screen and, after you enter your Exchange Password, you can create a new PIN. You can also create a new PIN at any time in Touchdown from the main screen. Press Menu>Change PIN and enter your Exchange password. You can create a new PIN.
If you do not see the option to Recover PIN or Create PIN, your MDM policies may be suppressing this option. In that case you must uninstall and reinstall Touchdown to create a new PIN.