Which command is most commonly used for changing a user's password in Linux?

The most commonly used command for changing a user's password in Linux is the passwd command. This utility is straightforward and designed specifically for password management tasks. When executed, it allows a user to set or update their password securely, often prompting for the current password before allowing the new password to be set. This command is widely recognized and included in virtually all Linux distributions, adhering to standard user management practices.

In contrast, the other terms listed do not serve the function of changing a user's password. The term "change" is too generic and does not correspond to a specific command within the Linux environment. "Update" is also a broad term that could apply to various actions in computing but does not refer to password management. Lastly, "mod" typically refers to modifications in certain contexts but is not associated with changing user passwords. Therefore, passwd stands out as the specific command intended for managing user passwords in Linux.