The net user command is a really easy command to use. Just execute it like this:
net user username password
...replacing username with your Windows 7 account name, and password with your desired new password.
For example, I might change my password to n3verE@Tsn0W by executing the net user command this way:
net user Tim n3verE@Tsn0W
Assuming everything was executed properly, you should have been greeted with a The command completed successfully. message after pressing Enter.
Tip: There are spaces between net, user, the username, and the password. If your username has a space, like Tim Fisher, use quotes. Reusing my example above but with my full name as my username, I would have executed net user "Tim Fisher" n3verE@Tsn0W.
Not Sure What Your Username Is?
If you were the last person to logon to Windows 7 before forgetting your password, your username should be listed right there on the logon screen. You can see Tim in big, bold letters in the screenshot above.
However, if you're resetting the password to another user on the computer and aren't sure exactly what the username is, you can generate a list by executing the net user command without options, for example:
A list of all of the users on the computer will be generated right in the Command Prompt window, which you can then reference for proper spelling when changing the password as described above.