A nice and easy one, but one that i must admit have fallen foul to a couple of times.
If your worried your going to break something then take a snapshot of the applicance prior to all this.
1. Start the appliance up and press the space bar to freeze the GRUB menu. You need to be fairly quick on this bit (usually takes me a couple of times)
2. Press e to enable edit mode.
3. Append rw init=/bin/bash to the end of the line
4. Press F10 to reboot. The appliance will now boot up to a shell
5. Type passwd to change the root password. Type it twice and press Enter to confirm.
6. Reboot the appliance using the power options from the VMRC or vSphere client menu, or type reboot (which ever takes your fancy).
And your done…. (if you took a snapshot do your tests then dont forget to remove it)