This is the most common technique used, what I try when i get an Ubuntu System with un-protected Recovery Mode GRUB entries. Read this from howtogeek.com
First a quick look into how to reset password via the Recovery mode.
"Esc" to get to the menu as shown below :
"drop to shell prompt" and if it still asks for a password, then you can try this.
Select the Recovery mode entry and press "e" to edit the entry.
Then in next screen select the Kernel entry. You are presented as an entry like this.
Once you have done this hit "Enter" and use "b" to boot
You are presented with a root shell
Issue "passwd" command to change password for root.
Issue "passwd <username>" to change password for a user.
Issue "sync" command to write the changes to disk.