usermod -u 500 -o username to change the user ID back to 500.
Note that changing a user ID doesn't “give the user root permissions”. What it actually does is to make the user name another name for user 0, i.e. the root user. A user is defined by the user ID. It's possible, but very rarely useful, to have multiple names for the same user ID. Furthermore, using the root account (i.e. user ID 0, by whatever name) for normal work is a bad idea: you increase the risk of breaking your system, you don't actually get significant benefits, and some programs will flatly refuse to work.
To “give a user root permissions”, the most common method is to use sudo. Run the program
visudo and add a line to the file like
jack ALL = (ALL:ALL) ALL
Then the user
jack will be able to run commands as root by adding
sudo in front of them, or using graphical sudo wrappers.
Note that a lot of software doesn't support special characters such as spaces in user names. The user name (the one in the first column of
/etc/passwd is a machine-parseable name and should only contain ASCII letters and digits. The fifth field contains the full name, i.e. the human-readable name.