The function user-full-name with no arguments returns the value of the variable user-full-name. This variable is initialised:

• to the value of the environment variable NAME if it is defined; or
• to the Gecos field of the passwd entry.

Therefore, you can do one of the following:

• set the Gecos field of your passwd entry using one of the chfn or usermod command;
• set your user name in the NAME environment variable in your shell's initialisation file;
• set the user-full-name variable in your Emacs' initialisation file.

The function user-full-name with no arguments returns the value of the variable user-full-name. This variable is initialised:

• to the value of the environment variable NAME if it is defined; or
• to the Gecos field of the passwd entry.

Therefore, you can do one of the following:

• set the Gecos field of your passwd entry using one of the chfn or usermod commands;
• set your user name in the NAME environment variable in your shell's initialisation file;
• set the user-full-name variable in your Emacs' initialisation file.