How to add a startup script on Debian linux

1) create your script (myscript) and add it to /etc/init.d/

2) make it executable: chmod 755 myscript

3) add the appropriate symbolic links: update-rc.d myscript defaults

If you want to remove the script from startup do the following:

$ update-rc.d -f myscript remove

Then remove the script from the init.d directory.

Reference

How to change username in Ubuntu

In order to change your old username (old_user) to a new one (new_user) do the following:
(I tried this logged in as a different user)

$ usermod -l new_user old_user

Now change the name of the home directories:

$ mv /home/old_user /home/new_user

Next, edit the /etc/passwd file, look for new_user and change the home directory to the new one.

Now, reboot your computer.