Final answer:
The 'passwd' command is used to change the user's password on Unix/Linux systems. It ensures security by requiring the old password before allowing a new one to be set.
Step-by-step explanation:
The command that can be used to change a user's password in most Unix/Linux systems is passwd. When a user wants to update or change their password, they would open a terminal or command prompt and simply type this command followed by pressing Enter. This would then prompt the user to enter their old password for security purposes, followed by the new password. It's important to choose a strong password that includes a mix of letters, numbers, and symbols to enhance account security.