This blog describes how to change or reset OS admin password and security password.

To Reset OS Admin or Security Password

Step 1. From Vsphere client, login to CUCM console through this username and passowrd: pwrecovery/pwreset. You get screen, as shown in the image:         
                                                                                        

Step 2. In order to connect to cd/dvd drive, you can select any iso image from datastore. This does not re-image your CUCM but this is just to check CUCM connectivity with vm client.     
                          

Step 3. After the iso image is selected, it asks you to press any key. Once it test then you get a prompt to remove the iso.                                                                                                             

Step 4. Press any key to continue. you get the option to reset OS Admin password (enter a) or Security password (enter s).                                                                                          

Step 5. You can change the password accordingly for OS Admin or Security.                                                                                                                                                                                  

Note: Please take fresh backup after changing Security password to avoid any backup/restore related issues.