how can I change what the command "apt-get update" looks for when executing the update?

Most of the time, a source line will be in the file /etc/apt/sources.list, so you should edit that. However, if you don't find it there, look at files inside the directory /etc/apt/sources.list.d. As far as I know, a source line must be on of those two places. Reference: man sources.list.

Putting the comment character # in front of any source line should be enough for apt to ignore it.

There should be a line with launchpad somewhere. This should show where:

grep -Ri launchpad /etc/apt/sources.list*

Could be also that you have configured a personalized path for sources.lists:

grep -R Dir /etc/apt/apt.conf*

Find it and remove it.